diff --git a/CHANGELOG.md b/CHANGELOG.md
index 35e48e7564..fb9322a103 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [0.7.0-beta.0] - Unreleased
 
+### Added
+
+- Added support for integer parts-per-second timeunit, with `badarg` raised on int64 overflow
+
 ## [0.7.0-alpha.1] - 2026-04-06
 
 ### Added
diff --git a/libs/estdlib/src/erlang.erl b/libs/estdlib/src/erlang.erl
index 07f00469f2..b0f92e771b 100644
--- a/libs/estdlib/src/erlang.erl
+++ b/libs/estdlib/src/erlang.erl
@@ -189,7 +189,7 @@
 -type atom_encoding() :: latin1 | utf8 | unicode.
 
 -type mem_type() :: binary.
--type time_unit() :: second | millisecond | microsecond | nanosecond | native.
+-type time_unit() :: second | millisecond | microsecond | nanosecond | native | pos_integer().
 -type timestamp() :: {
     MegaSecs :: non_neg_integer(), Secs :: non_neg_integer(), MicroSecs :: non_neg_integer
 }.
diff --git a/libs/exavmlib/lib/System.ex b/libs/exavmlib/lib/System.ex
index 2e819d3f23..1395ba1734 100644
--- a/libs/exavmlib/lib/System.ex
+++ b/libs/exavmlib/lib/System.ex
@@ -26,6 +26,7 @@ defmodule System do
           | :millisecond
           | :microsecond
           | :nanosecond
+          | pos_integer()
 
   @doc """
   Returns the current monotonic time in the `:native` time unit.
diff --git a/src/libAtomVM/nifs.c b/src/libAtomVM/nifs.c
index 3c0722972b..ed0a1d5b67 100644
--- a/src/libAtomVM/nifs.c
+++ b/src/libAtomVM/nifs.c
@@ -1841,41 +1841,122 @@ term nif_erlang_make_ref_0(Context *ctx, int argc, term argv[])
     return term_from_ref_ticks(ref_ticks, &ctx->heap);
 }
 
-term nif_erlang_monotonic_time_1(Context *ctx, int argc, term argv[])
+static bool time_unit_to_parts_per_second(term unit, avm_int64_t *parts_per_second)
 {
-    UNUSED(ctx);
-
-    term unit;
-    if (argc == 0) {
-        unit = NATIVE_ATOM;
+    if (unit == SECOND_ATOM) {
+        *parts_per_second = 1;
+    } else if (unit == MILLISECOND_ATOM) {
+        *parts_per_second = 1000;
+    } else if (unit == MICROSECOND_ATOM) {
+        *parts_per_second = INT64_C(1000000);
+    } else if (unit == NANOSECOND_ATOM || unit == NATIVE_ATOM) {
+        *parts_per_second = INT64_C(1000000000);
+    } else if (term_is_int64(unit)) {
+        *parts_per_second = term_maybe_unbox_int64(unit);
+        if (UNLIKELY(*parts_per_second <= 0)) {
+            return false;
+        }
     } else {
-        unit = argv[0];
+        return false;
     }
 
-    struct timespec ts;
-    sys_monotonic_time(&ts);
+    return true;
+}
 
-    if (unit == SECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ts.tv_sec);
+// Convert nanoseconds to parts using: parts = nanoseconds * pps / 1e9
+// Splits into high/low to avoid intermediate overflow.
+// Caller must ensure 0 <= nanoseconds < 1e9 and pps > 0.
+static bool nanoseconds_to_parts_per_second(
+    avm_int64_t nanoseconds, avm_int64_t parts_per_second, bool round_up, avm_int64_t *parts)
+{
+    avm_int64_t quotient = parts_per_second / INT64_C(1000000000);
+    avm_int64_t remainder = parts_per_second % INT64_C(1000000000);
+    avm_int64_t fractional_high = nanoseconds * quotient;
+    avm_int64_t remainder_product = nanoseconds * remainder;
+    avm_int64_t fractional_low = remainder_product / INT64_C(1000000000);
 
-    } else if (unit == MILLISECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * 1000UL + ts.tv_nsec / 1000000UL);
+    if (round_up && (remainder_product % INT64_C(1000000000)) != 0) {
+        fractional_low += 1;
+    }
 
-    } else if (unit == MICROSECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * 1000000UL + ts.tv_nsec / 1000UL);
+    if (UNLIKELY(fractional_high > INT64_MAX - fractional_low)) {
+        return false;
+    }
 
-    } else if (unit == NANOSECOND_ATOM || unit == NATIVE_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * INT64_C(1000000000) + ts.tv_nsec);
+    *parts = fractional_high + fractional_low;
+    return true;
+}
 
-    } else {
-        RAISE_ERROR(BADARG_ATOM);
+// Convert a normalized timespec (0 <= tv_nsec < 1e9) to integer parts.
+// Uses floor semantics for negative timestamps with non-zero tv_nsec.
+static bool timespec_to_parts_per_second(
+    const struct timespec *ts, avm_int64_t parts_per_second, avm_int64_t *parts)
+{
+    avm_int64_t seconds = (avm_int64_t) ts->tv_sec;
+    avm_int64_t fractional_part;
+
+    if (ts->tv_nsec == 0 || seconds >= 0) {
+        if (UNLIKELY(
+                ((seconds > 0) && (seconds > INT64_MAX / parts_per_second))
+                || ((seconds < 0) && (seconds < INT64_MIN / parts_per_second)))) {
+            return false;
+        }
+
+        if (UNLIKELY(!nanoseconds_to_parts_per_second(
+                (avm_int64_t) ts->tv_nsec, parts_per_second, false, &fractional_part))) {
+            return false;
+        }
+
+        avm_int64_t second_part = seconds * parts_per_second;
+        if (UNLIKELY(second_part > INT64_MAX - fractional_part)) {
+            return false;
+        }
+
+        *parts = second_part + fractional_part;
+        return true;
+    }
+
+    // Preserve floor semantics for normalized negative timespecs such as {-2, 999999999}.
+    avm_int64_t adjusted_seconds = seconds + 1;
+    if (UNLIKELY(adjusted_seconds < INT64_MIN / parts_per_second)) {
+        return false;
+    }
+
+    if (UNLIKELY(!nanoseconds_to_parts_per_second(
+            INT64_C(1000000000) - (avm_int64_t) ts->tv_nsec, parts_per_second, true,
+            &fractional_part))) {
+        return false;
     }
+
+    avm_int64_t second_part = adjusted_seconds * parts_per_second;
+    if (UNLIKELY(second_part < INT64_MIN + fractional_part)) {
+        return false;
+    }
+
+    *parts = second_part - fractional_part;
+    return true;
 }
 
-term nif_erlang_system_time_1(Context *ctx, int argc, term argv[])
+static term make_time_in_unit(Context *ctx, term unit, void (*time_fun)(struct timespec *))
 {
-    UNUSED(ctx);
+    avm_int64_t parts_per_second;
+    if (UNLIKELY(!time_unit_to_parts_per_second(unit, &parts_per_second))) {
+        RAISE_ERROR(BADARG_ATOM);
+    }
+
+    struct timespec ts;
+    time_fun(&ts);
+
+    avm_int64_t value;
+    if (UNLIKELY(!timespec_to_parts_per_second(&ts, parts_per_second, &value))) {
+        RAISE_ERROR(BADARG_ATOM);
+    }
 
+    return make_maybe_boxed_int64(ctx, value);
+}
+
+term nif_erlang_monotonic_time_1(Context *ctx, int argc, term argv[])
+{
     term unit;
     if (argc == 0) {
         unit = NATIVE_ATOM;
@@ -1883,24 +1964,19 @@ term nif_erlang_system_time_1(Context *ctx, int argc, term argv[])
         unit = argv[0];
     }
 
-    struct timespec ts;
-    sys_time(&ts);
-
-    if (unit == SECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ts.tv_sec);
-
-    } else if (unit == MILLISECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * 1000UL + ts.tv_nsec / 1000000UL);
-
-    } else if (unit == MICROSECOND_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * 1000000UL + ts.tv_nsec / 1000UL);
-
-    } else if (unit == NANOSECOND_ATOM || unit == NATIVE_ATOM) {
-        return make_maybe_boxed_int64(ctx, ((int64_t) ts.tv_sec) * INT64_C(1000000000) + ts.tv_nsec);
+    return make_time_in_unit(ctx, unit, sys_monotonic_time);
+}
 
+term nif_erlang_system_time_1(Context *ctx, int argc, term argv[])
+{
+    term unit;
+    if (argc == 0) {
+        unit = NATIVE_ATOM;
     } else {
-        RAISE_ERROR(BADARG_ATOM);
+        unit = argv[0];
     }
+
+    return make_time_in_unit(ctx, unit, sys_time);
 }
 
 static term build_datetime_from_tm(Context *ctx, struct tm *broken_down_time)
@@ -2007,33 +2083,26 @@ term nif_erlang_timestamp_0(Context *ctx, int argc, term argv[])
 
 term nif_calendar_system_time_to_universal_time_2(Context *ctx, int argc, term argv[])
 {
-    UNUSED(ctx);
     UNUSED(argc);
 
-    struct timespec ts;
-
+    if (UNLIKELY(!term_is_int64(argv[0]))) {
+        RAISE_ERROR(BADARG_ATOM);
+    }
     avm_int64_t value = term_maybe_unbox_int64(argv[0]);
 
-    if (argv[1] == SECOND_ATOM) {
-        ts.tv_sec = (time_t) value;
-        ts.tv_nsec = 0;
-
-    } else if (argv[1] == MILLISECOND_ATOM) {
-        ts.tv_sec = (time_t) (value / 1000);
-        ts.tv_nsec = (value % 1000) * 1000000;
-
-    } else if (argv[1] == MICROSECOND_ATOM) {
-        ts.tv_sec = (time_t) (value / 1000000);
-        ts.tv_nsec = (value % 1000000) * 1000;
-
-    } else if (argv[1] == NANOSECOND_ATOM || argv[1] == NATIVE_ATOM) {
-        ts.tv_sec = (time_t) (value / INT64_C(1000000000));
-        ts.tv_nsec = value % INT64_C(1000000000);
-
-    } else {
+    avm_int64_t parts_per_second;
+    if (UNLIKELY(!time_unit_to_parts_per_second(argv[1], &parts_per_second))) {
         RAISE_ERROR(BADARG_ATOM);
     }
 
+    // Floor division: round negative fractional seconds toward negative infinity
+    avm_int64_t quotient = value / parts_per_second;
+    avm_int64_t remainder = value % parts_per_second;
+    struct timespec ts = {
+        .tv_sec = (time_t) (quotient - (remainder < 0)),
+        .tv_nsec = 0
+    };
+
     struct tm broken_down_time;
     return build_datetime_from_tm(ctx, gmtime_r(&ts.tv_sec, &broken_down_time));
 }
diff --git a/tests/erlang_tests/test_monotonic_time.erl b/tests/erlang_tests/test_monotonic_time.erl
index 52c1a390c1..980b0cd91f 100644
--- a/tests/erlang_tests/test_monotonic_time.erl
+++ b/tests/erlang_tests/test_monotonic_time.erl
@@ -38,6 +38,9 @@ start() ->
     true = is_integer(N2 - N1) andalso (N2 - N1) >= 0,
 
     ok = test_native_monotonic_time(),
+    ok = test_integer_time_unit(),
+    ok = test_non_power_of_10_integer_time_unit(),
+    ok = test_bad_integer_time_unit(),
 
     1.
 
@@ -46,6 +49,15 @@ test_diff(X) when is_integer(X) andalso X >= 0 ->
 test_diff(X) when X < 0 ->
     0.
 
+expect(F, Expect) ->
+    try
+        F(),
+        fail
+    catch
+        _:E when E == Expect ->
+            ok
+    end.
+
 test_native_monotonic_time() ->
     Na1 = erlang:monotonic_time(native),
     receive
@@ -54,3 +66,55 @@ test_native_monotonic_time() ->
     Na2 = erlang:monotonic_time(native),
     true = is_integer(Na2 - Na1) andalso (Na2 - Na1) >= 0,
     ok.
+
+test_integer_time_unit() ->
+    %% integer 1 = parts per second, equivalent to second
+    S = erlang:monotonic_time(second),
+    S1 = erlang:monotonic_time(1),
+    true = abs(S1 - S) =< 1,
+
+    %% integer 1000 = parts per second, equivalent to millisecond
+    Ms = erlang:monotonic_time(millisecond),
+    Ms1 = erlang:monotonic_time(1000),
+    true = abs(Ms1 - Ms) =< 1,
+
+    %% integer 1000000 = parts per second, equivalent to microsecond
+    Us = erlang:monotonic_time(microsecond),
+    Us1 = erlang:monotonic_time(1000000),
+    true = abs(Us1 - Us) =< 1000,
+
+    %% integer 1000000000 = parts per second, equivalent to nanosecond
+    Ns = erlang:monotonic_time(nanosecond),
+    Ns1 = erlang:monotonic_time(1000000000),
+    true = abs(Ns1 - Ns) =< 1000000,
+
+    %% verify monotonicity with integer unit
+    T1 = erlang:monotonic_time(1000),
+    receive
+    after 1 -> ok
+    end,
+    T2 = erlang:monotonic_time(1000),
+    true = T2 >= T1,
+
+    ok.
+
+test_non_power_of_10_integer_time_unit() ->
+    ok = test_integer_time_unit_monotonicity(256),
+    ok = test_integer_time_unit_monotonicity(48000),
+    ok.
+
+test_bad_integer_time_unit() ->
+    ok = expect(fun() -> erlang:monotonic_time(0) end, badarg),
+    ok = expect(fun() -> erlang:monotonic_time(-1) end, badarg),
+    ok.
+
+test_integer_time_unit_monotonicity(PartsPerSecond) ->
+    T1 = erlang:monotonic_time(PartsPerSecond),
+    receive
+    after 1 -> ok
+    end,
+    T2 = erlang:monotonic_time(PartsPerSecond),
+    true = is_integer(T1),
+    true = is_integer(T2),
+    true = T2 >= T1,
+    ok.
diff --git a/tests/erlang_tests/test_system_time.erl b/tests/erlang_tests/test_system_time.erl
index 423c7aed6f..0c864d9f52 100644
--- a/tests/erlang_tests/test_system_time.erl
+++ b/tests/erlang_tests/test_system_time.erl
@@ -34,9 +34,15 @@ start() ->
     ok = test_os_system_time(),
     ok = test_time_unit_ratios(),
 
+    ok = test_integer_time_unit(),
+    ok = test_non_power_of_10_integer_time_unit(),
+    ok = test_bad_integer_time_unit(),
+
     ok = expect(fun() -> erlang:system_time(not_a_time_unit) end, badarg),
 
     ok = test_system_time_to_universal_time(),
+    ok = test_integer_unit_universal_time(),
+    ok = test_bad_integer_unit_universal_time(),
 
     0.
 
@@ -126,6 +132,15 @@ test_system_time_to_universal_time() ->
     {{2023, 7, 8}, {20, 19, 39}} = calendar:system_time_to_universal_time(1688847579, second),
 
     {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, second),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, millisecond),
+    {{1969, 12, 31}, {23, 59, 58}} = calendar:system_time_to_universal_time(-1001, millisecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, microsecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, nanosecond),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, native),
+
+    ok = expect(
+        fun() -> calendar:system_time_to_universal_time(not_an_integer, second) end, badarg
+    ),
 
     ok = test_nanosecond_universal_time(),
     ok = test_native_universal_time(),
@@ -165,3 +180,87 @@ test_native_universal_time() ->
     {{1970, 1, 1}, {0, 0, 0}} = calendar:system_time_to_universal_time(0, native),
     {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(1000000000, native),
     ok.
+
+test_integer_time_unit() ->
+    %% integer 1 = parts per second, equivalent to second
+    S = erlang:system_time(second),
+    S1 = erlang:system_time(1),
+    true = abs(S1 - S) =< 1,
+
+    %% integer 1000 = parts per second, equivalent to millisecond
+    Ms = erlang:system_time(millisecond),
+    Ms1 = erlang:system_time(1000),
+    true = abs(Ms1 - Ms) =< 1,
+
+    %% integer 1000000 = parts per second, equivalent to microsecond
+    Us = erlang:system_time(microsecond),
+    Us1 = erlang:system_time(1000000),
+    true = abs(Us1 - Us) =< 1000,
+
+    %% integer 1000000000 = parts per second, equivalent to nanosecond
+    Ns = erlang:system_time(nanosecond),
+    Ns1 = erlang:system_time(1000000000),
+    true = abs(Ns1 - Ns) =< 1000000,
+
+    %% verify values are positive
+    true = S1 > 0,
+    true = Ms1 > 0,
+    true = Us1 > 0,
+    true = Ns1 > 0,
+
+    ok.
+
+test_non_power_of_10_integer_time_unit() ->
+    ok = test_integer_parts_per_second_ratio(256),
+    ok = test_integer_parts_per_second_ratio(48000),
+    ok.
+
+test_integer_unit_universal_time() ->
+    %% integer 1 = seconds
+    {{1970, 1, 1}, {0, 0, 0}} = calendar:system_time_to_universal_time(0, 1),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(1, 1),
+    {{2023, 7, 8}, {20, 19, 39}} = calendar:system_time_to_universal_time(1688847579, 1),
+
+    %% integer 1000 = milliseconds
+    {{1970, 1, 1}, {0, 0, 0}} = calendar:system_time_to_universal_time(0, 1000),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(1000, 1000),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(1001, 1000),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, 1000),
+    {{1969, 12, 31}, {23, 59, 58}} = calendar:system_time_to_universal_time(-1001, 1000),
+
+    %% integer 1000000 = microseconds
+    {{1970, 1, 1}, {0, 0, 0}} = calendar:system_time_to_universal_time(0, 1000000),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(1000000, 1000000),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, 1000000),
+
+    %% integer 256 and 48000 = arbitrary parts per second
+    {{1970, 1, 1}, {0, 0, 0}} = calendar:system_time_to_universal_time(255, 256),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(256, 256),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, 256),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-255, 256),
+    {{1970, 1, 1}, {0, 0, 1}} = calendar:system_time_to_universal_time(48000, 48000),
+    {{1969, 12, 31}, {23, 59, 59}} = calendar:system_time_to_universal_time(-1, 48000),
+
+    ok.
+
+test_bad_integer_time_unit() ->
+    ok = expect(fun() -> erlang:system_time(0) end, badarg),
+    ok = expect(fun() -> erlang:system_time(-1) end, badarg),
+    ok.
+
+test_bad_integer_unit_universal_time() ->
+    ok = expect(
+        fun() -> calendar:system_time_to_universal_time(not_an_integer, second) end, badarg
+    ),
+    ok = expect(fun() -> calendar:system_time_to_universal_time(not_an_integer, 1000) end, badarg),
+    ok = expect(fun() -> calendar:system_time_to_universal_time(0, 0) end, badarg),
+    ok = expect(fun() -> calendar:system_time_to_universal_time(0, -1) end, badarg),
+    ok.
+
+test_integer_parts_per_second_ratio(PartsPerSecond) ->
+    Seconds = erlang:system_time(second),
+    Parts = erlang:system_time(PartsPerSecond),
+    true = is_integer(Parts) andalso Parts > 0,
+    true = Parts >= Seconds * PartsPerSecond,
+    true = Parts < Seconds * PartsPerSecond + (PartsPerSecond * 2),
+    ok.