diff --git a/lib/tzdata/data_loader.ex b/lib/tzdata/data_loader.ex
index 192dd54..cf63948 100644
--- a/lib/tzdata/data_loader.ex
+++ b/lib/tzdata/data_loader.ex
@@ -81,10 +81,10 @@ defmodule Tzdata.DataLoader do
 
   defp do_latest_file_size_by_head({:error, error}), do: {:error, error}
 
-  defp do_latest_file_size_by_head({_tag, resp_code, _headers}) when resp_code != 200,
+  defp do_latest_file_size_by_head({:ok, {resp_code, _headers}}) when resp_code != 200,
     do: {:error, :did_not_get_ok_response}
 
-  defp do_latest_file_size_by_head({_tag, _resp_code, headers}) do
+  defp do_latest_file_size_by_head({:ok, {_resp_code, headers}}) do
     headers
     |> content_length_from_headers
   end
diff --git a/mix.exs b/mix.exs
index 78d1e6f..27db004 100644
--- a/mix.exs
+++ b/mix.exs
@@ -28,7 +28,9 @@ defmodule Tzdata.Mixfile do
   defp deps do
     [
       {:hackney, "~> 1.17"},
-      {:ex_doc, "~> 0.21", only: :dev, runtime: false}
+      {:ex_doc, "~> 0.21", only: :dev, runtime: false},
+      {:sham, "~> 1.2", only: :test},
+      {:bandit, "~> 1.0", only: :test}
     ]
   end
 
diff --git a/mix.lock b/mix.lock
index c278e8a..3803e8b 100644
--- a/mix.lock
+++ b/mix.lock
@@ -1,16 +1,25 @@
 %{
+  "bandit": {:hex, :bandit, "1.10.2", "d15ea32eb853b5b42b965b24221eb045462b2ba9aff9a0bda71157c06338cbff", [:mix], [{:hpax, "~> 1.0", [hex: :hpax, repo: "hexpm", optional: false]}, {:plug, "~> 1.18", [hex: :plug, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}, {:thousand_island, "~> 1.0", [hex: :thousand_island, repo: "hexpm", optional: false]}, {:websock, "~> 0.5", [hex: :websock, repo: "hexpm", optional: false]}], "hexpm", "27b2a61b647914b1726c2ced3601473be5f7aa6bb468564a688646a689b3ee45"},
   "certifi": {:hex, :certifi, "2.14.0", "ed3bef654e69cde5e6c022df8070a579a79e8ba2368a00acf3d75b82d9aceeed", [:rebar3], [], "hexpm", "ea59d87ef89da429b8e905264fdec3419f84f2215bb3d81e07a18aac919026c3"},
   "earmark_parser": {:hex, :earmark_parser, "1.4.43", "34b2f401fe473080e39ff2b90feb8ddfeef7639f8ee0bbf71bb41911831d77c5", [:mix], [], "hexpm", "970a3cd19503f5e8e527a190662be2cee5d98eed1ff72ed9b3d1a3d466692de8"},
   "ex_doc": {:hex, :ex_doc, "0.37.2", "2a3aa7014094f0e4e286a82aa5194a34dd17057160988b8509b15aa6c292720c", [:mix], [{:earmark_parser, "~> 1.4.42", [hex: :earmark_parser, repo: "hexpm", optional: false]}, {:makeup_c, ">= 0.1.0", [hex: :makeup_c, repo: "hexpm", optional: true]}, {:makeup_elixir, "~> 0.14 or ~> 1.0", [hex: :makeup_elixir, repo: "hexpm", optional: false]}, {:makeup_erlang, "~> 0.1 or ~> 1.0", [hex: :makeup_erlang, repo: "hexpm", optional: false]}, {:makeup_html, ">= 0.1.0", [hex: :makeup_html, repo: "hexpm", optional: true]}], "hexpm", "4dfa56075ce4887e4e8b1dcc121cd5fcb0f02b00391fd367ff5336d98fa49049"},
   "hackney": {:hex, :hackney, "1.23.0", "55cc09077112bcb4a69e54be46ed9bc55537763a96cd4a80a221663a7eafd767", [:rebar3], [{:certifi, "~> 2.14.0", [hex: :certifi, repo: "hexpm", optional: false]}, {:idna, "~> 6.1.0", [hex: :idna, repo: "hexpm", optional: false]}, {:metrics, "~> 1.0.0", [hex: :metrics, repo: "hexpm", optional: false]}, {:mimerl, "~> 1.1", [hex: :mimerl, repo: "hexpm", optional: false]}, {:parse_trans, "3.4.1", [hex: :parse_trans, repo: "hexpm", optional: false]}, {:ssl_verify_fun, "~> 1.1.0", [hex: :ssl_verify_fun, repo: "hexpm", optional: false]}, {:unicode_util_compat, "~> 0.7.0", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "6cd1c04cd15c81e5a493f167b226a15f0938a84fc8f0736ebe4ddcab65c0b44e"},
+  "hpax": {:hex, :hpax, "1.0.3", "ed67ef51ad4df91e75cc6a1494f851850c0bd98ebc0be6e81b026e765ee535aa", [:mix], [], "hexpm", "8eab6e1cfa8d5918c2ce4ba43588e894af35dbd8e91e6e55c817bca5847df34a"},
   "idna": {:hex, :idna, "6.1.1", "8a63070e9f7d0c62eb9d9fcb360a7de382448200fbbd1b106cc96d3d8099df8d", [:rebar3], [{:unicode_util_compat, "~>0.7.0", [hex: :unicode_util_compat, repo: "hexpm", optional: false]}], "hexpm", "92376eb7894412ed19ac475e4a86f7b413c1b9fbb5bd16dccd57934157944cea"},
   "makeup": {:hex, :makeup, "1.2.1", "e90ac1c65589ef354378def3ba19d401e739ee7ee06fb47f94c687016e3713d1", [:mix], [{:nimble_parsec, "~> 1.4", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "d36484867b0bae0fea568d10131197a4c2e47056a6fbe84922bf6ba71c8d17ce"},
   "makeup_elixir": {:hex, :makeup_elixir, "1.0.1", "e928a4f984e795e41e3abd27bfc09f51db16ab8ba1aebdba2b3a575437efafc2", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}, {:nimble_parsec, "~> 1.2.3 or ~> 1.3", [hex: :nimble_parsec, repo: "hexpm", optional: false]}], "hexpm", "7284900d412a3e5cfd97fdaed4f5ed389b8f2b4cb49efc0eb3bd10e2febf9507"},
   "makeup_erlang": {:hex, :makeup_erlang, "1.0.2", "03e1804074b3aa64d5fad7aa64601ed0fb395337b982d9bcf04029d68d51b6a7", [:mix], [{:makeup, "~> 1.0", [hex: :makeup, repo: "hexpm", optional: false]}], "hexpm", "af33ff7ef368d5893e4a267933e7744e46ce3cf1f61e2dccf53a111ed3aa3727"},
   "metrics": {:hex, :metrics, "1.0.1", "25f094dea2cda98213cecc3aeff09e940299d950904393b2a29d191c346a8486", [:rebar3], [], "hexpm", "69b09adddc4f74a40716ae54d140f93beb0fb8978d8636eaded0c31b6f099f16"},
+  "mime": {:hex, :mime, "2.0.7", "b8d739037be7cd402aee1ba0306edfdef982687ee7e9859bee6198c1e7e2f128", [:mix], [], "hexpm", "6171188e399ee16023ffc5b76ce445eb6d9672e2e241d2df6050f3c771e80ccd"},
   "mimerl": {:hex, :mimerl, "1.3.0", "d0cd9fc04b9061f82490f6581e0128379830e78535e017f7780f37fea7545726", [:rebar3], [], "hexpm", "a1e15a50d1887217de95f0b9b0793e32853f7c258a5cd227650889b38839fe9d"},
   "nimble_parsec": {:hex, :nimble_parsec, "1.4.2", "8efba0122db06df95bfaa78f791344a89352ba04baedd3849593bfce4d0dc1c6", [:mix], [], "hexpm", "4b21398942dda052b403bbe1da991ccd03a053668d147d53fb8c4e0efe09c973"},
   "parse_trans": {:hex, :parse_trans, "3.4.1", "6e6aa8167cb44cc8f39441d05193be6e6f4e7c2946cb2759f015f8c56b76e5ff", [:rebar3], [], "hexpm", "620a406ce75dada827b82e453c19cf06776be266f5a67cff34e1ef2cbb60e49a"},
+  "plug": {:hex, :plug, "1.19.1", "09bac17ae7a001a68ae393658aa23c7e38782be5c5c00c80be82901262c394c0", [:mix], [{:mime, "~> 1.0 or ~> 2.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2 or ~> 2.0", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4.3 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "560a0017a8f6d5d30146916862aaf9300b7280063651dd7e532b8be168511e62"},
+  "plug_crypto": {:hex, :plug_crypto, "2.1.1", "19bda8184399cb24afa10be734f84a16ea0a2bc65054e23a62bb10f06bc89491", [:mix], [], "hexpm", "6470bce6ffe41c8bd497612ffde1a7e4af67f36a15eea5f921af71cf3e11247c"},
+  "sham": {:hex, :sham, "1.2.5", "aa1b41b042d9e3a44c08a09006ef9ebacb60802d3bb868a3c4d21463884706e1", [:mix], [{:bandit, "~> 1.0", [hex: :bandit, repo: "hexpm", optional: true]}, {:plug_cowboy, "~> 2.0", [hex: :plug_cowboy, repo: "hexpm", optional: true]}], "hexpm", "4de55b7b78c9a35254bd470f8bfa8e0dbd8cf4a1f7152defd7932a3675b8fdf9"},
   "ssl_verify_fun": {:hex, :ssl_verify_fun, "1.1.7", "354c321cf377240c7b8716899e182ce4890c5938111a1296add3ec74cf1715df", [:make, :mix, :rebar3], [], "hexpm", "fe4c190e8f37401d30167c8c405eda19469f34577987c76dde613e838bbc67f8"},
+  "telemetry": {:hex, :telemetry, "1.3.0", "fedebbae410d715cf8e7062c96a1ef32ec22e764197f70cda73d82778d61e7a2", [:rebar3], [], "hexpm", "7015fc8919dbe63764f4b4b87a95b7c0996bd539e0d499be6ec9d7f3875b79e6"},
+  "thousand_island": {:hex, :thousand_island, "1.4.3", "2158209580f633be38d43ec4e3ce0a01079592b9657afff9080d5d8ca149a3af", [:mix], [{:telemetry, "~> 0.4 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "6e4ce09b0fd761a58594d02814d40f77daff460c48a7354a15ab353bb998ea0b"},
   "unicode_util_compat": {:hex, :unicode_util_compat, "0.7.0", "bc84380c9ab48177092f43ac89e4dfa2c6d62b40b8bd132b1059ecc7232f9a78", [:rebar3], [], "hexpm", "25eee6d67df61960cf6a794239566599b09e17e668d3700247bc498638152521"},
+  "websock": {:hex, :websock, "0.5.3", "2f69a6ebe810328555b6fe5c831a851f485e303a7c8ce6c5f675abeb20ebdadc", [:mix], [], "hexpm", "6105453d7fac22c712ad66fab1d45abdf049868f253cf719b625151460b8b453"},
 }
diff --git a/test/data_loader_test.exs b/test/data_loader_test.exs
new file mode 100644
index 0000000..6249cdd
--- /dev/null
+++ b/test/data_loader_test.exs
@@ -0,0 +1,69 @@
+defmodule Tzdata.DataLoaderTest do
+  use ExUnit.Case, async: true
+
+  alias Tzdata.DataLoader
+
+  setup do
+    sham = Sham.start()
+    original_client = Application.get_env(:tzdata, :http_client)
+
+    Application.put_env(:tzdata, :http_client, Tzdata.HTTPClient.Hackney)
+
+    on_exit(fn ->
+      if original_client do
+        Application.put_env(:tzdata, :http_client, original_client)
+      else
+        Application.delete_env(:tzdata, :http_client)
+      end
+    end)
+
+    {:ok, sham: sham}
+  end
+
+  describe "latest_file_size/1" do
+    test "gets file size from HEAD request", %{sham: sham} do
+      url = "http://localhost:#{sham.port}/test.tar.gz"
+
+      Sham.expect(sham, "HEAD", "/test.tar.gz", fn conn ->
+        # For HEAD requests, include a body matching the content-length
+        # (in reality, HEAD responses don't include the body, but for testing
+        # with Sham, Plug will calculate content-length from the body)
+        body = String.duplicate("x", 12345)
+
+        conn
+        |> Plug.Conn.send_resp(200, body)
+      end)
+
+      assert {:ok, 12345} = DataLoader.latest_file_size(url)
+    end
+
+    test "falls back to GET when HEAD fails", %{sham: sham} do
+      url = "http://localhost:#{sham.port}/fallback.tar.gz"
+
+      Sham.expect(sham, "HEAD", "/fallback.tar.gz", fn conn ->
+        Plug.Conn.resp(conn, 404, "")
+      end)
+
+      Sham.expect(sham, "GET", "/fallback.tar.gz", fn conn ->
+        Plug.Conn.resp(conn, 200, "test body content")
+      end)
+
+      assert {:ok, 17} = DataLoader.latest_file_size(url)
+    end
+  end
+
+  describe "last_modified_of_latest_available/1" do
+    test "gets last modified date from HEAD request", %{sham: sham} do
+      url = "http://localhost:#{sham.port}/modified.tar.gz"
+
+      Sham.expect(sham, "HEAD", "/modified.tar.gz", fn conn ->
+        conn
+        |> Plug.Conn.put_resp_header("last-modified", "Wed, 21 Oct 2015 07:28:00 GMT")
+        |> Plug.Conn.send_resp(200, "")
+      end)
+
+      assert {:ok, "Wed, 21 Oct 2015 07:28:00 GMT"} =
+               DataLoader.last_modified_of_latest_available(url)
+    end
+  end
+end