parse ISOz to determine seconds offset for expiration time

lib/ex_aws/config/auth_cache.ex

@@ -1,6 +1,10 @@
 defmodule ExAws.Config.AuthCache do
   use GenServer
 
+  @moduledoc false
+
+  # http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html
+
   def start_link(opts \\ []) do
     GenServer.start_link(__MODULE__, :ok, opts)
   end
@@ -37,9 +41,8 @@ defmodule ExAws.Config.AuthCache do
   end
 
   def refresh_in(expiration) do
-    expiration = Timex.DateFormat.parse!(expiration, "{ISOz}")
-    |> Timex.Date.convert(:secs)
-    time_to_expiration = expiration - Timex.Date.now(:secs)
+    expiration = expiration |> ExAws.Utils.iso_z_to_secs
+    time_to_expiration = expiration - ExAws.Utils.now_in_seconds
     refresh_in = time_to_expiration - 2 * 60 # check two min prior to expiration
     refresh_in * 1000
   end

lib/ex_aws/utils.ex

@@ -47,4 +47,29 @@ defmodule ExAws.Utils do
   def upcase(value) when is_binary(value) do
     String.upcase(value)
   end
+
+  @seconds_0_to_1970 :calendar.datetime_to_gregorian_seconds({{1970, 1, 1}, {0, 0, 0}})
+
+  def iso_z_to_secs(<<date::binary-10, "T", time::binary-8, "Z">>) do
+    <<year::binary-4, "-", mon::binary-2, "-", day::binary-2>> = date
+    <<hour::binary-2, ":", min::binary-2, ":", sec::binary-2>> = time
+    year = year |> String.to_integer
+    mon  = mon  |> String.to_integer
+    day  = day  |> String.to_integer
+    hour = hour |> String.to_integer
+    min  = min  |> String.to_integer
+    sec  = sec  |> String.to_integer
+
+    # Seriously? Gregorian seconds but not epoch seconds?
+    greg_secs = :calendar.datetime_to_gregorian_seconds({{year, mon, day}, {hour, min, sec}})
+    greg_secs - @seconds_0_to_1970
+  end
+
+  def now_in_seconds do
+    greg_secs = :os.timestamp
+    |> :calendar.now_to_universal_time
+    |> :calendar.datetime_to_gregorian_seconds
+
+    greg_secs - @seconds_0_to_1970
+  end
 end

test/lib/ex_aws/utils_test.exs

@@ -24,4 +24,8 @@ defmodule ExAws.UtilsTest do
     assert %{"FooBar" => ["foo", "bar"]} == [foo_bar: ["foo", "bar"]]
     |> camelize_keys(deep: true)
   end
+
+  test "iso_z_to_secs/1" do
+    assert iso_z_to_secs("2015-07-05T22:16:18Z") == 1436134578
+  end
 end