From d87e1b66fb53dea089935ea28dff150412237733 Mon Sep 17 00:00:00 2001
From: GTR1701 <dawidkowal04@gmail.com>
Date: Sat, 31 Jan 2026 14:06:20 +0100
Subject: [PATCH 1/4] refactor(calendar): move from custom dates to date-fns

---
 .../components/arc-academic-semester-card.tsx |  8 ++--
 .../components/arc-day-button.tsx             | 16 ++++----
 .../components/arc-event-card.tsx             | 22 ++++++++--
 .../components/arc-internal.tsx               | 40 +++++++------------
 .../types/internal.ts                         | 10 -----
 .../utils/academic-calendar-mapper.tsx        |  7 ++--
 .../utils/event-calendar-mapper.tsx           |  5 ++-
 .../utils/find-existing-day-swap.ts           |  5 ++-
 .../utils/format-date-object.ts               |  6 ---
 .../utils/format-time.ts                      | 16 --------
 .../utils/get-events-for-day.ts               | 14 +++----
 .../utils/get-month-by-date.ts                | 14 -------
 .../utils/get-month-by-number-and-year.ts     | 10 -----
 .../utils/serialize-date-day.ts               |  5 ---
 14 files changed, 59 insertions(+), 119 deletions(-)
 delete mode 100644 src/features/abstract-resource-calendar/utils/format-date-object.ts
 delete mode 100644 src/features/abstract-resource-calendar/utils/format-time.ts
 delete mode 100644 src/features/abstract-resource-calendar/utils/get-month-by-date.ts
 delete mode 100644 src/features/abstract-resource-calendar/utils/get-month-by-number-and-year.ts
 delete mode 100644 src/features/abstract-resource-calendar/utils/serialize-date-day.ts

diff --git a/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx b/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
index 7abcf32d..e292d117 100644
--- a/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
@@ -1,5 +1,7 @@
 "use client";
 
+import { format } from "date-fns";
+
 import {
   DeleteButtonWithDialog,
   EditButton,
@@ -7,8 +9,6 @@ import {
 } from "@/features/resources";
 import type { ResourceDataType } from "@/features/resources/types";
 
-import { serializeDateDay } from "../utils/serialize-date-day";
-
 export function AcademicSemesterCard({
   semester,
   clickable,
@@ -21,8 +21,8 @@ export function AcademicSemesterCard({
       <section className="my-auto">
         <p className="font-medium">{semester.name}</p>
         <p className="mt-1 text-xs">
-          {serializeDateDay(semester.semesterStartDate)} –{" "}
-          {serializeDateDay(semester.examSessionLastDate)}
+          {format(new Date(semester.semesterStartDate), "yyyy-MM-dd")} –{" "}
+          {format(semester.examSessionLastDate, "yyyy-MM-dd")}
         </p>
       </section>
       {clickable ? (
diff --git a/src/features/abstract-resource-calendar/components/arc-day-button.tsx b/src/features/abstract-resource-calendar/components/arc-day-button.tsx
index 5215b923..b8a354e9 100644
--- a/src/features/abstract-resource-calendar/components/arc-day-button.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-day-button.tsx
@@ -1,28 +1,28 @@
 import { Button } from "@/components/ui/button";
 import { cn } from "@/lib/utils";
 
-import type { DateObject, EventCardType } from "../types/internal";
+import type { EventCardType } from "../types/internal";
 import { EventBadge } from "./arc-event-badge";
 
 export function DayButton({
-  day,
-  today,
+  dayButtonDate,
   clickable,
   eventsForDay,
   currentDate,
   onDayClick,
 }: {
-  day: number;
-  today: DateObject;
+  dayButtonDate: Date;
   clickable: boolean;
   currentDate: Date;
   eventsForDay: EventCardType[];
   onDayClick: () => void;
 }) {
   const isCurrentDay =
-    day === currentDate.getDate() &&
-    today.month.value === currentDate.getMonth() + 1 &&
-    today.year === currentDate.getFullYear();
+    dayButtonDate.getDate() === currentDate.getDate() &&
+    dayButtonDate.getMonth() === currentDate.getMonth() &&
+    dayButtonDate.getFullYear() === currentDate.getFullYear();
+
+  const day = dayButtonDate.getDate();
 
   return (
     <Button
diff --git a/src/features/abstract-resource-calendar/components/arc-event-card.tsx b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
index e62b6878..65b63551 100644
--- a/src/features/abstract-resource-calendar/components/arc-event-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
@@ -1,5 +1,8 @@
 "use client";
 
+import { formatDate, isSameDay } from "date-fns";
+import { pl } from "date-fns/locale";
+
 import type { Resource } from "@/features/resources";
 import { DeleteButtonWithDialog, EditButton } from "@/features/resources";
 import type {
@@ -7,8 +10,6 @@ import type {
   ResourceDataType,
 } from "@/features/resources/types";
 
-import { formatTime } from "../utils/format-time";
-
 export function EventCard({
   event,
   resource,
@@ -27,11 +28,24 @@ export function EventCard({
       <header>
         <p className="font-medium">{event.name}</p>
         <p className="mt-1 text-xs">
-          {formatTime(event, "startTime")}
+          {formatDate(
+            event.startTime,
+            isSameDay(event.startTime, event.endTime) ? "HH:mm" : "HH:mm (E)",
+            { locale: pl },
+          )}
           {event.startTime === event.endTime ? (
             ""
           ) : (
-            <>—{formatTime(event, "endTime")}</>
+            <>
+              —
+              {formatDate(
+                event.endTime,
+                isSameDay(event.startTime, event.endTime)
+                  ? "HH:mm"
+                  : "HH:mm (E)",
+                { locale: pl },
+              )}
+            </>
           )}
         </p>
       </header>
diff --git a/src/features/abstract-resource-calendar/components/arc-internal.tsx b/src/features/abstract-resource-calendar/components/arc-internal.tsx
index 05885bb2..63a43a8c 100644
--- a/src/features/abstract-resource-calendar/components/arc-internal.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-internal.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { parse } from "date-fns";
+import { format, getDaysInMonth, parse } from "date-fns";
 import { ChevronLeft, ChevronRight } from "lucide-react";
 
 import { Link } from "@/components/core/link";
@@ -9,9 +9,7 @@ import type { SearchParameters } from "@/types/components";
 
 import { WEEKDAYS } from "../constants";
 import type { MappedCalendarData } from "../types/internal";
-import { formatDateObject } from "../utils/format-date-object";
 import { getEventsForDay } from "../utils/get-events-for-day";
-import { getMonthByNumberAndYear } from "../utils/get-month-by-number-and-year";
 import { getMonthLink } from "../utils/get-month-link";
 import { DayButton } from "./arc-day-button";
 
@@ -27,23 +25,17 @@ export function CalendarInternal({
   onDayClick: (dayKey: string) => void;
 }) {
   const { year, month } = searchParams;
-  const currentDate = new Date();
+  const todayDate = new Date();
   const displayedYear = (
-    year == null ? currentDate : parse(year, "yyyy", currentDate)
+    year == null ? todayDate : parse(year, "yyyy", todayDate)
   ).getFullYear();
   const displayedMonth =
-    (month == null ? currentDate : parse(month, "MM", currentDate)).getMonth() +
-    1;
+    (month == null ? todayDate : parse(month, "MM", todayDate)).getMonth() + 1;
 
-  const currentDisplayedMonth = getMonthByNumberAndYear(
-    displayedMonth,
-    displayedYear,
-  );
-
-  const firstDayOfMonth = new Date(displayedYear, displayedMonth - 1, 1);
-  const startDayOfWeek = (firstDayOfMonth.getDay() + 6) % 7;
+  const currentDisplayedMonth = new Date(displayedYear, displayedMonth - 1, 1);
+  const startDayOfWeek = (currentDisplayedMonth.getDay() + 6) % 7;
+  const totalCells = startDayOfWeek + getDaysInMonth(currentDisplayedMonth);
 
-  const totalCells = startDayOfWeek + currentDisplayedMonth.daysInMonth;
   const calendarDays = Array.from({ length: totalCells }, (_, index) =>
     index < startDayOfWeek
       ? ({
@@ -66,7 +58,7 @@ export function CalendarInternal({
           </Link>
         </Button>
         <span className="min-w-[200px]">
-          {currentDisplayedMonth.name} {displayedYear}
+          {format(currentDisplayedMonth, "MMMM")} {displayedYear}
         </span>
         <Button variant="ghost" size="icon" aria-label="Next month" asChild>
           <Link href={getMonthLink(displayedYear, displayedMonth + 1)}>
@@ -88,23 +80,19 @@ export function CalendarInternal({
         if (cell.type === "empty") {
           return <div key={cell.id} />;
         }
-        const displayedDateObject = {
-          year: displayedYear,
-          month: currentDisplayedMonth,
-          day: cell.day,
-        };
-        const dayEvents = getEventsForDay(displayedDateObject, mappedData);
+
+        const cellDate = new Date(currentDisplayedMonth.setDate(cell.day));
+        const dayEvents = getEventsForDay(cellDate, mappedData);
 
         return (
           <DayButton
             key={cell.id}
-            day={cell.day}
-            today={displayedDateObject}
+            dayButtonDate={cellDate}
             clickable={clickable}
             eventsForDay={dayEvents}
-            currentDate={currentDate}
+            currentDate={todayDate}
             onDayClick={() => {
-              onDayClick(formatDateObject(displayedDateObject));
+              onDayClick(format(cellDate, "yyyy-MM-dd"));
             }}
           />
         );
diff --git a/src/features/abstract-resource-calendar/types/internal.ts b/src/features/abstract-resource-calendar/types/internal.ts
index 038d002f..ccb6b3ea 100644
--- a/src/features/abstract-resource-calendar/types/internal.ts
+++ b/src/features/abstract-resource-calendar/types/internal.ts
@@ -7,16 +7,6 @@ import type {
   ResourcePk,
 } from "@/features/resources/types";
 
-export interface DateObject {
-  year: number;
-  month: {
-    value: number;
-    name: string;
-    daysInMonth: number;
-  };
-  day: number;
-}
-
 export interface ResourceCalendarProps<T extends Resource> {
   resource: T;
 }
diff --git a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
index f6a1c439..92f09adb 100644
--- a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
+++ b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
@@ -1,3 +1,5 @@
+import { format } from "date-fns";
+
 import type { Resource } from "@/features/resources";
 import type { ResourceDataWithRelations } from "@/features/resources/types";
 
@@ -5,7 +7,6 @@ import { AcademicSemesterCard } from "../components/arc-academic-semester-card";
 import { DaySwapCard } from "../components/arc-day-swap-card";
 import { HolidayCard } from "../components/arc-holiday-card";
 import type { MappedCalendarData } from "../types/internal";
-import { serializeDateDay } from "./serialize-date-day";
 
 export function academicCalendarMapper(
   semesters: ResourceDataWithRelations<Resource.AcademicSemesters>[],
@@ -43,7 +44,7 @@ export function academicCalendarMapper(
         currentDate.getTime() <= lastDate.getTime();
         currentDate.setDate(currentDate.getDate() + 1)
       ) {
-        dayKeys.push(serializeDateDay(currentDate.toISOString()));
+        dayKeys.push(format(currentDate.toISOString(), "yyyy-MM-dd"));
       }
 
       const holidayCard = (
@@ -71,7 +72,7 @@ export function academicCalendarMapper(
     }
 
     for (const daySwap of daySwaps) {
-      const dayKey = serializeDateDay(daySwap.date);
+      const dayKey = format(daySwap.date, "yyyy-MM-dd");
 
       mappedData.dayEvents[dayKey] ??= [];
 
diff --git a/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx b/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
index c6890a01..bd0e15b1 100644
--- a/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
+++ b/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
@@ -1,9 +1,10 @@
+import { format } from "date-fns";
+
 import { Resource } from "@/features/resources";
 import type { ResourceDataType } from "@/features/resources/types";
 
 import { EventCard } from "../components/arc-event-card";
 import type { MappedCalendarData } from "../types/internal";
-import { serializeDateDay } from "./serialize-date-day";
 
 export function eventCalendarMapper(
   events: ResourceDataType<Resource.CalendarEvents>[],
@@ -24,7 +25,7 @@ export function eventCalendarMapper(
       currentDate.getTime() <= lastDate.getTime();
       currentDate.setDate(currentDate.getDate() + 1)
     ) {
-      dayKeys.push(serializeDateDay(currentDate.toISOString()));
+      dayKeys.push(format(currentDate.toISOString(), "yyyy-MM-dd"));
     }
 
     const eventCard = (
diff --git a/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts b/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
index 3f4ae333..dc2fa04d 100644
--- a/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
+++ b/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
@@ -1,11 +1,12 @@
+import { format } from "date-fns";
+
 import type { SemesterStructure } from "../types/internal";
-import { serializeDateDay } from "./serialize-date-day";
 
 export function findExistingDaySwap(
   date: string,
   parentSemester: SemesterStructure,
 ): boolean {
-  const dayKey = serializeDateDay(date);
+  const dayKey = format(date, "yyyy-MM-dd");
   const semesterEventsForDay = parentSemester.semesterEvents[dayKey];
 
   if (semesterEventsForDay == null) {
diff --git a/src/features/abstract-resource-calendar/utils/format-date-object.ts b/src/features/abstract-resource-calendar/utils/format-date-object.ts
deleted file mode 100644
index 0cbb115f..00000000
--- a/src/features/abstract-resource-calendar/utils/format-date-object.ts
+++ /dev/null
@@ -1,6 +0,0 @@
-import type { DateObject } from "../types/internal";
-
-export function formatDateObject(displayedDateObject: DateObject): string {
-  const { year, month, day } = displayedDateObject;
-  return `${year.toString()}-${month.value.toString().padStart(2, "0")}-${day.toString().padStart(2, "0")}`;
-}
diff --git a/src/features/abstract-resource-calendar/utils/format-time.ts b/src/features/abstract-resource-calendar/utils/format-time.ts
deleted file mode 100644
index dca72d50..00000000
--- a/src/features/abstract-resource-calendar/utils/format-time.ts
+++ /dev/null
@@ -1,16 +0,0 @@
-import { formatDate, isSameDay } from "date-fns";
-import { pl } from "date-fns/locale";
-
-import type { Resource } from "@/features/resources";
-import type { ResourceDataType } from "@/features/resources/types";
-
-export function formatTime(
-  event: ResourceDataType<Resource.CalendarEvents>,
-  timeType: "startTime" | "endTime",
-) {
-  return formatDate(
-    event[timeType],
-    isSameDay(event.startTime, event.endTime) ? "HH:mm" : "HH:mm (E)",
-    { locale: pl },
-  );
-}
diff --git a/src/features/abstract-resource-calendar/utils/get-events-for-day.ts b/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
index 57a16096..c114a18e 100644
--- a/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
+++ b/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
@@ -1,16 +1,12 @@
-import type {
-  DateObject,
-  EventCardType,
-  MappedCalendarData,
-} from "../types/internal";
-import { formatDateObject } from "./format-date-object";
+import { format } from "date-fns";
+
+import type { EventCardType, MappedCalendarData } from "../types/internal";
 
 export function getEventsForDay(
-  dateObject: DateObject,
+  date: Date,
   events: MappedCalendarData,
 ): EventCardType[] {
-  const formattedDateObject = formatDateObject(dateObject);
-  const dayEvents = events.dayEvents[formattedDateObject] ?? [];
+  const dayEvents = events.dayEvents[format(date, "yyyy-MM-dd")] ?? [];
 
   return dayEvents;
 }
diff --git a/src/features/abstract-resource-calendar/utils/get-month-by-date.ts b/src/features/abstract-resource-calendar/utils/get-month-by-date.ts
deleted file mode 100644
index 110aa246..00000000
--- a/src/features/abstract-resource-calendar/utils/get-month-by-date.ts
+++ /dev/null
@@ -1,14 +0,0 @@
-export function getMonthByDate(
-  date: Date,
-  locale = "pl",
-): { value: number; name: string; daysInMonth: number } {
-  const monthNumber = date.getMonth() + 1;
-  const year = date.getFullYear();
-  const monthName = date.toLocaleString(locale, { month: "long" });
-  const daysInMonth = new Date(year, monthNumber, 0).getDate();
-  return {
-    value: monthNumber,
-    name: monthName,
-    daysInMonth,
-  };
-}
diff --git a/src/features/abstract-resource-calendar/utils/get-month-by-number-and-year.ts b/src/features/abstract-resource-calendar/utils/get-month-by-number-and-year.ts
deleted file mode 100644
index d09411b0..00000000
--- a/src/features/abstract-resource-calendar/utils/get-month-by-number-and-year.ts
+++ /dev/null
@@ -1,10 +0,0 @@
-import { getMonthByDate } from "./get-month-by-date";
-
-export function getMonthByNumberAndYear(
-  month: number,
-  year: number,
-  locale = "pl",
-): { value: number; name: string; daysInMonth: number } {
-  const date = new Date(year, month - 1);
-  return getMonthByDate(date, locale);
-}
diff --git a/src/features/abstract-resource-calendar/utils/serialize-date-day.ts b/src/features/abstract-resource-calendar/utils/serialize-date-day.ts
deleted file mode 100644
index b047f923..00000000
--- a/src/features/abstract-resource-calendar/utils/serialize-date-day.ts
+++ /dev/null
@@ -1,5 +0,0 @@
-import { format } from "date-fns";
-
-export function serializeDateDay(date: Date | string): string {
-  return format(new Date(date), "yyyy-MM-dd");
-}

From 2e5f98b98125162bd8ae3b3bcc4bce88ac3d0d48 Mon Sep 17 00:00:00 2001
From: GTR1701 <dawidkowal04@gmail.com>
Date: Sat, 31 Jan 2026 14:48:19 +0100
Subject: [PATCH 2/4] refactor(calendar): change displayed dates to a more user
 friendly format

---
 .../components/arc-event-card.tsx             | 20 +++++--------------
 .../components/arc-holiday-card.tsx           | 13 +++++++++++-
 .../components/arc-internal.tsx               |  3 ++-
 .../utils/get-modal-header.test.ts            |  6 +++++-
 .../utils/get-modal-header.ts                 | 10 +++++++---
 5 files changed, 31 insertions(+), 21 deletions(-)

diff --git a/src/features/abstract-resource-calendar/components/arc-event-card.tsx b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
index 65b63551..6994af7a 100644
--- a/src/features/abstract-resource-calendar/components/arc-event-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
@@ -1,6 +1,6 @@
 "use client";
 
-import { formatDate, isSameDay } from "date-fns";
+import { formatDate } from "date-fns";
 import { pl } from "date-fns/locale";
 
 import type { Resource } from "@/features/resources";
@@ -28,23 +28,13 @@ export function EventCard({
       <header>
         <p className="font-medium">{event.name}</p>
         <p className="mt-1 text-xs">
-          {formatDate(
-            event.startTime,
-            isSameDay(event.startTime, event.endTime) ? "HH:mm" : "HH:mm (E)",
-            { locale: pl },
-          )}
           {event.startTime === event.endTime ? (
-            ""
+            <>{formatDate(event.startTime, "HH:mm", { locale: pl })}</>
           ) : (
             <>
-              —
-              {formatDate(
-                event.endTime,
-                isSameDay(event.startTime, event.endTime)
-                  ? "HH:mm"
-                  : "HH:mm (E)",
-                { locale: pl },
-              )}
+              {formatDate(event.endTime, "EEEE HH:mm", { locale: pl })}
+              {" — "}
+              {formatDate(event.endTime, "EEEE HH:mm", { locale: pl })}
             </>
           )}
         </p>
diff --git a/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx b/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
index 1a0ce0c5..33b10339 100644
--- a/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
@@ -1,5 +1,8 @@
 "use client";
 
+import { formatDate } from "date-fns";
+import { pl } from "date-fns/locale";
+
 import { useArfSheet } from "@/features/abstract-resource-form";
 import type { ArfSheetFormProps } from "@/features/abstract-resource-form/types";
 import { Resource } from "@/features/resources";
@@ -35,7 +38,15 @@ export function HolidayCard({
     >
       <div className="my-auto">
         <div className="font-medium">{event.description}</div>
-        <div className="mt-1 text-xs">{event.startDate}</div>
+        {event.startDate === event.lastDate ? (
+          <>{formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}</>
+        ) : (
+          <>
+            {formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}
+            {" — "}
+            {formatDate(event.lastDate, "dd MMMM yyyy", { locale: pl })}
+          </>
+        )}
       </div>
     </SheetCard>
   );
diff --git a/src/features/abstract-resource-calendar/components/arc-internal.tsx b/src/features/abstract-resource-calendar/components/arc-internal.tsx
index 63a43a8c..ffdd2774 100644
--- a/src/features/abstract-resource-calendar/components/arc-internal.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-internal.tsx
@@ -1,6 +1,7 @@
 "use client";
 
 import { format, getDaysInMonth, parse } from "date-fns";
+import { pl } from "date-fns/locale";
 import { ChevronLeft, ChevronRight } from "lucide-react";
 
 import { Link } from "@/components/core/link";
@@ -58,7 +59,7 @@ export function CalendarInternal({
           </Link>
         </Button>
         <span className="min-w-[200px]">
-          {format(currentDisplayedMonth, "MMMM")} {displayedYear}
+          {format(currentDisplayedMonth, "LLLL yyyy", { locale: pl })}
         </span>
         <Button variant="ghost" size="icon" aria-label="Next month" asChild>
           <Link href={getMonthLink(displayedYear, displayedMonth + 1)}>
diff --git a/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts b/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
index 78a3b170..46b1a3a5 100644
--- a/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
+++ b/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
@@ -1,3 +1,5 @@
+import { format } from "date-fns";
+import { pl } from "date-fns/locale";
 import { describe, expect, it } from "vitest";
 
 import { Resource } from "@/features/resources";
@@ -13,7 +15,9 @@ describe("getModalHeader", () => {
   });
 
   describe("when clickedDay is provided", () => {
-    const clickedDay = "2024-03-15";
+    const clickedDay = format(new Date("2024-03-15"), "dd MMMM yyyy", {
+      locale: pl,
+    });
 
     it("should return resource name with date for resource without relations", () => {
       const result = getModalHeader(Resource.CalendarEvents, clickedDay);
diff --git a/src/features/abstract-resource-calendar/utils/get-modal-header.ts b/src/features/abstract-resource-calendar/utils/get-modal-header.ts
index 03637bf5..ff13c2d7 100644
--- a/src/features/abstract-resource-calendar/utils/get-modal-header.ts
+++ b/src/features/abstract-resource-calendar/utils/get-modal-header.ts
@@ -1,3 +1,6 @@
+import { format } from "date-fns";
+import { pl } from "date-fns/locale";
+
 import { GrammaticalCase, declineNoun } from "@/features/polish";
 import { getResourceRelationDefinitions } from "@/features/resources";
 import type { Resource } from "@/features/resources";
@@ -21,6 +24,7 @@ export function getModalHeader(
       }),
     );
   } else {
+    const clickedDayDate = new Date(clickedDay);
     return relatedResources.length > 0
       ? relatedResources.length === 1
         ? `${toTitleCase(
@@ -28,7 +32,7 @@ export function getModalHeader(
               case: GrammaticalCase.Nominative,
               plural: true,
             }),
-          )} ${clickedDay}`
+          )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`
         : `${toTitleCase(
             relatedResources
               .map((relatedResource) =>
@@ -38,12 +42,12 @@ export function getModalHeader(
                 }),
               )
               .join(" i "),
-          )} ${clickedDay}`
+          )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`
       : `${toTitleCase(
           declineNoun(resource, {
             case: GrammaticalCase.Nominative,
             plural: true,
           }),
-        )} ${clickedDay}`;
+        )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`;
   }
 }

From e5bdaf7b557b85cd61f0456347bbdb9cbc95a8c7 Mon Sep 17 00:00:00 2001
From: GTR1701 <dawidkowal04@gmail.com>
Date: Thu, 19 Feb 2026 12:13:25 +0100
Subject: [PATCH 3/4] fix(arc): ensure date key integrity

---
 .../components/arc-academic-semester-card.tsx          |  9 +++++----
 .../components/arc-event-card.tsx                      |  2 +-
 .../components/arc-internal.tsx                        |  3 ++-
 .../utils/academic-calendar-mapper.tsx                 |  7 +++----
 .../utils/event-calendar-mapper.tsx                    |  5 ++---
 .../utils/find-existing-day-swap.ts                    |  5 ++---
 .../utils/format-date-key.ts                           |  3 +++
 .../utils/get-events-for-day.ts                        |  5 ++---
 .../utils/get-modal-header.ts                          | 10 ++++++----
 9 files changed, 26 insertions(+), 23 deletions(-)
 create mode 100644 src/features/abstract-resource-calendar/utils/format-date-key.ts

diff --git a/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx b/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
index e292d117..76fe2fec 100644
--- a/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-academic-semester-card.tsx
@@ -1,7 +1,5 @@
 "use client";
 
-import { format } from "date-fns";
-
 import {
   DeleteButtonWithDialog,
   EditButton,
@@ -9,6 +7,8 @@ import {
 } from "@/features/resources";
 import type { ResourceDataType } from "@/features/resources/types";
 
+import { formatDateKey } from "../utils/format-date-key";
+
 export function AcademicSemesterCard({
   semester,
   clickable,
@@ -21,8 +21,9 @@ export function AcademicSemesterCard({
       <section className="my-auto">
         <p className="font-medium">{semester.name}</p>
         <p className="mt-1 text-xs">
-          {format(new Date(semester.semesterStartDate), "yyyy-MM-dd")} –{" "}
-          {format(semester.examSessionLastDate, "yyyy-MM-dd")}
+          {formatDateKey(new Date(semester.semesterStartDate))}
+          {" — "}
+          {formatDateKey(new Date(semester.examSessionLastDate))}
         </p>
       </section>
       {clickable ? (
diff --git a/src/features/abstract-resource-calendar/components/arc-event-card.tsx b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
index 6994af7a..2a59256e 100644
--- a/src/features/abstract-resource-calendar/components/arc-event-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
@@ -29,7 +29,7 @@ export function EventCard({
         <p className="font-medium">{event.name}</p>
         <p className="mt-1 text-xs">
           {event.startTime === event.endTime ? (
-            <>{formatDate(event.startTime, "HH:mm", { locale: pl })}</>
+            formatDate(event.startTime, "HH:mm", { locale: pl })
           ) : (
             <>
               {formatDate(event.endTime, "EEEE HH:mm", { locale: pl })}
diff --git a/src/features/abstract-resource-calendar/components/arc-internal.tsx b/src/features/abstract-resource-calendar/components/arc-internal.tsx
index ffdd2774..eb079596 100644
--- a/src/features/abstract-resource-calendar/components/arc-internal.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-internal.tsx
@@ -10,6 +10,7 @@ import type { SearchParameters } from "@/types/components";
 
 import { WEEKDAYS } from "../constants";
 import type { MappedCalendarData } from "../types/internal";
+import { formatDateKey } from "../utils/format-date-key";
 import { getEventsForDay } from "../utils/get-events-for-day";
 import { getMonthLink } from "../utils/get-month-link";
 import { DayButton } from "./arc-day-button";
@@ -93,7 +94,7 @@ export function CalendarInternal({
             eventsForDay={dayEvents}
             currentDate={todayDate}
             onDayClick={() => {
-              onDayClick(format(cellDate, "yyyy-MM-dd"));
+              onDayClick(formatDateKey(cellDate));
             }}
           />
         );
diff --git a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
index 92f09adb..9327069a 100644
--- a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
+++ b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
@@ -1,5 +1,3 @@
-import { format } from "date-fns";
-
 import type { Resource } from "@/features/resources";
 import type { ResourceDataWithRelations } from "@/features/resources/types";
 
@@ -7,6 +5,7 @@ import { AcademicSemesterCard } from "../components/arc-academic-semester-card";
 import { DaySwapCard } from "../components/arc-day-swap-card";
 import { HolidayCard } from "../components/arc-holiday-card";
 import type { MappedCalendarData } from "../types/internal";
+import { formatDateKey } from "./format-date-key";
 
 export function academicCalendarMapper(
   semesters: ResourceDataWithRelations<Resource.AcademicSemesters>[],
@@ -44,7 +43,7 @@ export function academicCalendarMapper(
         currentDate.getTime() <= lastDate.getTime();
         currentDate.setDate(currentDate.getDate() + 1)
       ) {
-        dayKeys.push(format(currentDate.toISOString(), "yyyy-MM-dd"));
+        dayKeys.push(formatDateKey(currentDate));
       }
 
       const holidayCard = (
@@ -72,7 +71,7 @@ export function academicCalendarMapper(
     }
 
     for (const daySwap of daySwaps) {
-      const dayKey = format(daySwap.date, "yyyy-MM-dd");
+      const dayKey = formatDateKey(new Date(daySwap.date));
 
       mappedData.dayEvents[dayKey] ??= [];
 
diff --git a/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx b/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
index bd0e15b1..a05ce89c 100644
--- a/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
+++ b/src/features/abstract-resource-calendar/utils/event-calendar-mapper.tsx
@@ -1,10 +1,9 @@
-import { format } from "date-fns";
-
 import { Resource } from "@/features/resources";
 import type { ResourceDataType } from "@/features/resources/types";
 
 import { EventCard } from "../components/arc-event-card";
 import type { MappedCalendarData } from "../types/internal";
+import { formatDateKey } from "./format-date-key";
 
 export function eventCalendarMapper(
   events: ResourceDataType<Resource.CalendarEvents>[],
@@ -25,7 +24,7 @@ export function eventCalendarMapper(
       currentDate.getTime() <= lastDate.getTime();
       currentDate.setDate(currentDate.getDate() + 1)
     ) {
-      dayKeys.push(format(currentDate.toISOString(), "yyyy-MM-dd"));
+      dayKeys.push(formatDateKey(currentDate));
     }
 
     const eventCard = (
diff --git a/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts b/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
index dc2fa04d..19bb36eb 100644
--- a/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
+++ b/src/features/abstract-resource-calendar/utils/find-existing-day-swap.ts
@@ -1,12 +1,11 @@
-import { format } from "date-fns";
-
 import type { SemesterStructure } from "../types/internal";
+import { formatDateKey } from "./format-date-key";
 
 export function findExistingDaySwap(
   date: string,
   parentSemester: SemesterStructure,
 ): boolean {
-  const dayKey = format(date, "yyyy-MM-dd");
+  const dayKey = formatDateKey(new Date(date));
   const semesterEventsForDay = parentSemester.semesterEvents[dayKey];
 
   if (semesterEventsForDay == null) {
diff --git a/src/features/abstract-resource-calendar/utils/format-date-key.ts b/src/features/abstract-resource-calendar/utils/format-date-key.ts
new file mode 100644
index 00000000..f2ebca47
--- /dev/null
+++ b/src/features/abstract-resource-calendar/utils/format-date-key.ts
@@ -0,0 +1,3 @@
+import { format } from "date-fns";
+
+export const formatDateKey = (date: Date) => format(date, "yyyy-MM-dd");
diff --git a/src/features/abstract-resource-calendar/utils/get-events-for-day.ts b/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
index c114a18e..e3595b05 100644
--- a/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
+++ b/src/features/abstract-resource-calendar/utils/get-events-for-day.ts
@@ -1,12 +1,11 @@
-import { format } from "date-fns";
-
 import type { EventCardType, MappedCalendarData } from "../types/internal";
+import { formatDateKey } from "./format-date-key";
 
 export function getEventsForDay(
   date: Date,
   events: MappedCalendarData,
 ): EventCardType[] {
-  const dayEvents = events.dayEvents[format(date, "yyyy-MM-dd")] ?? [];
+  const dayEvents = events.dayEvents[formatDateKey(date)] ?? [];
 
   return dayEvents;
 }
diff --git a/src/features/abstract-resource-calendar/utils/get-modal-header.ts b/src/features/abstract-resource-calendar/utils/get-modal-header.ts
index ff13c2d7..54688710 100644
--- a/src/features/abstract-resource-calendar/utils/get-modal-header.ts
+++ b/src/features/abstract-resource-calendar/utils/get-modal-header.ts
@@ -24,7 +24,9 @@ export function getModalHeader(
       }),
     );
   } else {
-    const clickedDayDate = new Date(clickedDay);
+    const clickedDayFormatted = format(new Date(clickedDay), "dd MMMM yyyy", {
+      locale: pl,
+    });
     return relatedResources.length > 0
       ? relatedResources.length === 1
         ? `${toTitleCase(
@@ -32,7 +34,7 @@ export function getModalHeader(
               case: GrammaticalCase.Nominative,
               plural: true,
             }),
-          )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`
+          )} ${clickedDayFormatted}`
         : `${toTitleCase(
             relatedResources
               .map((relatedResource) =>
@@ -42,12 +44,12 @@ export function getModalHeader(
                 }),
               )
               .join(" i "),
-          )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`
+          )} ${clickedDayFormatted}`
       : `${toTitleCase(
           declineNoun(resource, {
             case: GrammaticalCase.Nominative,
             plural: true,
           }),
-        )} ${format(clickedDayDate, "dd MMMM yyyy", { locale: pl })}`;
+        )} ${clickedDayFormatted}`;
   }
 }

From 6b63351491cc4ddc83aceceba5bcf19173af8bd6 Mon Sep 17 00:00:00 2001
From: GTR1701 <dawidkowal04@gmail.com>
Date: Wed, 25 Mar 2026 19:50:59 +0100
Subject: [PATCH 4/4] fix(arc): minor date issues

---
 .../components/arc-event-card.tsx             |  2 +-
 .../components/arc-holiday-card.tsx           | 20 ++++++++++---------
 .../components/arc-internal.tsx               |  2 +-
 .../utils/academic-calendar-mapper.tsx        |  2 +-
 .../utils/get-modal-header.test.ts            |  9 ++++++---
 5 files changed, 20 insertions(+), 15 deletions(-)

diff --git a/src/features/abstract-resource-calendar/components/arc-event-card.tsx b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
index 2a59256e..8877c8cf 100644
--- a/src/features/abstract-resource-calendar/components/arc-event-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-event-card.tsx
@@ -32,7 +32,7 @@ export function EventCard({
             formatDate(event.startTime, "HH:mm", { locale: pl })
           ) : (
             <>
-              {formatDate(event.endTime, "EEEE HH:mm", { locale: pl })}
+              {formatDate(event.startTime, "EEEE HH:mm", { locale: pl })}
               {" — "}
               {formatDate(event.endTime, "EEEE HH:mm", { locale: pl })}
             </>
diff --git a/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx b/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
index 33b10339..0672685e 100644
--- a/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-holiday-card.tsx
@@ -38,15 +38,17 @@ export function HolidayCard({
     >
       <div className="my-auto">
         <div className="font-medium">{event.description}</div>
-        {event.startDate === event.lastDate ? (
-          <>{formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}</>
-        ) : (
-          <>
-            {formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}
-            {" — "}
-            {formatDate(event.lastDate, "dd MMMM yyyy", { locale: pl })}
-          </>
-        )}
+        <div className="mt-1 text-xs">
+          {event.startDate === event.lastDate ? (
+            <>{formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}</>
+          ) : (
+            <>
+              {formatDate(event.startDate, "dd MMMM yyyy", { locale: pl })}
+              {" — "}
+              {formatDate(event.lastDate, "dd MMMM yyyy", { locale: pl })}
+            </>
+          )}
+        </div>
       </div>
     </SheetCard>
   );
diff --git a/src/features/abstract-resource-calendar/components/arc-internal.tsx b/src/features/abstract-resource-calendar/components/arc-internal.tsx
index eb079596..c695127d 100644
--- a/src/features/abstract-resource-calendar/components/arc-internal.tsx
+++ b/src/features/abstract-resource-calendar/components/arc-internal.tsx
@@ -83,7 +83,7 @@ export function CalendarInternal({
           return <div key={cell.id} />;
         }
 
-        const cellDate = new Date(currentDisplayedMonth.setDate(cell.day));
+        const cellDate = new Date(displayedYear, displayedMonth - 1, cell.day);
         const dayEvents = getEventsForDay(cellDate, mappedData);
 
         return (
diff --git a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
index 9327069a..5ccd6b68 100644
--- a/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
+++ b/src/features/abstract-resource-calendar/utils/academic-calendar-mapper.tsx
@@ -39,7 +39,7 @@ export function academicCalendarMapper(
 
       const dayKeys: string[] = [];
       for (
-        let currentDate = new Date(startDate);
+        let currentDate = startDate;
         currentDate.getTime() <= lastDate.getTime();
         currentDate.setDate(currentDate.getDate() + 1)
       ) {
diff --git a/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts b/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
index 46b1a3a5..c43c2576 100644
--- a/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
+++ b/src/features/abstract-resource-calendar/utils/get-modal-header.test.ts
@@ -4,6 +4,7 @@ import { describe, expect, it } from "vitest";
 
 import { Resource } from "@/features/resources";
 
+import { formatDateKey } from "./format-date-key";
 import { getModalHeader } from "./get-modal-header";
 
 describe("getModalHeader", () => {
@@ -15,18 +16,20 @@ describe("getModalHeader", () => {
   });
 
   describe("when clickedDay is provided", () => {
-    const clickedDay = format(new Date("2024-03-15"), "dd MMMM yyyy", {
+    const clickedDay = formatDateKey(new Date("2024-03-15"));
+
+    const formattedClickedDay = format(clickedDay, "dd MMMM yyyy", {
       locale: pl,
     });
 
     it("should return resource name with date for resource without relations", () => {
       const result = getModalHeader(Resource.CalendarEvents, clickedDay);
-      expect(result).toBe(`Wydarzenia kalendarzowe ${clickedDay}`);
+      expect(result).toBe(`Wydarzenia kalendarzowe ${formattedClickedDay}`);
     });
 
     it("should return single related resource name with date when one OneToMany relation exists", () => {
       const result = getModalHeader(Resource.AcademicSemesters, clickedDay);
-      expect(result).toBe(`Zamiany dni i święta ${clickedDay}`);
+      expect(result).toBe(`Zamiany dni i święta ${formattedClickedDay}`);
     });
   });
 });