diff --git a/README.md b/README.md
index a12841da..c22b15cc 100644
--- a/README.md
+++ b/README.md
@@ -25,7 +25,7 @@ If you wish to use instance roles to obtain AWS access keys you will need to add
 ```elixir
 def deps do
   [
-    ex_aws:    "~> 0.4.0",
+    ex_aws:    "~> 0.4.2",
     poison:    "~> 1.2.0",
     httpoison: "~> 0.7.0"
   ]
diff --git a/lib/ex_aws/auth.ex b/lib/ex_aws/auth.ex
index 858f9fec..a4d6a3fb 100644
--- a/lib/ex_aws/auth.ex
+++ b/lib/ex_aws/auth.ex
@@ -1,11 +1,10 @@
 defmodule ExAws.Auth do
   import ExAws.Auth.Utils
-  alias Timex.DateFormat
 
   @moduledoc false
 
   def headers(http_method, url, service, config, headers, body) do
-    now = %{Timex.Date.now | ms: 0}
+    now = :os.timestamp |> :calendar.now_to_universal_time
     headers = [
       {"host", URI.parse(url).host},
       {"x-amz-date", amz_date(now)} |
@@ -32,8 +31,8 @@ defmodule ExAws.Auth do
   end
   def handle_temp_credentials(headers, _), do: headers
 
-  def auth_header(access_key, secret_key, http_method, url, region, service, headers, body, now) do
-    date  = DateFormat.format!(now, "%Y%m%d", :strftime)
+  def auth_header(access_key, secret_key, http_method, url, region, service, headers, body, {date, _} = now) do
+    date  = date |> quasi_iso_format
     scope = "#{date}/#{region}/#{service}/aws4_request"
 
     signing_key = build_signing_key(secret_key, date, region, service)
@@ -59,8 +58,8 @@ defmodule ExAws.Auth do
   end
 
   def build_string_to_sign(canonical_request, now, scope) do
-    timestamp = now |> ExAws.Auth.Utils.amz_date
-    hashed_canonical_request = ExAws.Auth.Utils.hash_sha256(canonical_request)
+    timestamp = now |> amz_date
+    hashed_canonical_request = hash_sha256(canonical_request)
 
     [
       "AWS4-HMAC-SHA256", "\n",
diff --git a/lib/ex_aws/auth/utils.ex b/lib/ex_aws/auth/utils.ex
index 36d70227..95a1bec1 100644
--- a/lib/ex_aws/auth/utils.ex
+++ b/lib/ex_aws/auth/utils.ex
@@ -1,11 +1,4 @@
 defmodule ExAws.Auth.Utils do
-  def amz_date(now) do
-    now
-    |> Timex.DateFormat.format!("{ISOz}")
-    |> String.replace("-", "")
-    |> String.replace(":", "")
-  end
-
   def hash_sha256(data) do
     :sha256
     |> :crypto.hash(data)
@@ -29,4 +22,21 @@ defmodule ExAws.Auth.Utils do
     |> Atom.to_string
     |> String.upcase
   end
+
+  def amz_date({date, time}) do
+    date = date |> quasi_iso_format
+    time = time |> quasi_iso_format
+
+    [date, "T", time, "Z"]
+    |> IO.iodata_to_binary
+  end
+
+  def quasi_iso_format({y, m, d}) do
+    [y, m, d]
+    |> Enum.map(&Integer.to_string/1)
+    |> Enum.map(&zero_pad/1)
+  end
+
+  defp zero_pad(<<_>> = val), do: "0" <> val
+  defp zero_pad(val), do: val
 end
diff --git a/lib/ex_aws/config/auth_cache.ex b/lib/ex_aws/config/auth_cache.ex
index d0fe0746..36c42149 100644
--- a/lib/ex_aws/config/auth_cache.ex
+++ b/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
diff --git a/lib/ex_aws/utils.ex b/lib/ex_aws/utils.ex
index 8ece8e98..b2309566 100644
--- a/lib/ex_aws/utils.ex
+++ b/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
diff --git a/mix.exs b/mix.exs
index 1ca0619e..4ed2e76a 100644
--- a/mix.exs
+++ b/mix.exs
@@ -3,7 +3,7 @@ defmodule ExAws.Mixfile do
 
   def project do
     [app: :ex_aws,
-     version: "0.4.1",
+     version: "0.4.2",
      elixir: "~> 1.0",
      description: "AWS client. Currently supports Dynamo, Kinesis, Lambda, S3",
      name: "ExAws",
@@ -14,20 +14,17 @@ defmodule ExAws.Mixfile do
   end
 
   def application do
-    [applications: [:logger, :timex, :crypto],
+    [applications: [:logger, :crypto],
      mod: {ExAws, []}]
   end
 
   defp deps do
-    [
-      {:timex, "~> 0.13.4"},
-      {:sweet_xml, "~> 0.2.1", only: [:dev, :test]} |
-      deps(:test_dev)
-    ]
+    deps(:test_dev)
   end
 
   defp deps(:test_dev) do
     [
+      {:sweet_xml, "~> 0.2.1", only: [:dev, :test]},
       {:earmark, "~> 0.1", only: :dev},
       {:ex_doc, "~> 0.7", only: :dev},
       {:sweet_xml, "~> 0.2.1", only: [:test]},
diff --git a/mix.lock b/mix.lock
index a8118b2b..ffe3cd7d 100644
--- a/mix.lock
+++ b/mix.lock
@@ -9,5 +9,4 @@
   "mixunit": {:hex, :mixunit, "0.9.2"},
   "poison": {:hex, :poison, "1.2.1"},
   "ssl_verify_hostname": {:hex, :ssl_verify_hostname, "1.0.5"},
-  "sweet_xml": {:hex, :sweet_xml, "0.2.1"},
-  "timex": {:hex, :timex, "0.13.4"}}
+  "sweet_xml": {:hex, :sweet_xml, "0.2.1"}}
diff --git a/test/lib/ex_aws/auth/utils_test.exs b/test/lib/ex_aws/auth/utils_test.exs
new file mode 100644
index 00000000..22582585
--- /dev/null
+++ b/test/lib/ex_aws/auth/utils_test.exs
@@ -0,0 +1,15 @@
+defmodule ExAws.Auth.UtilsTest do
+  use ExUnit.Case, async: true
+  import ExAws.Auth.Utils
+
+  test "quasi_iso_date/1" do
+    assert quasi_iso_format({2015, 1, 2}) == ["2015", "01", "02"]
+    assert quasi_iso_format({2015, 11, 12}) == ["2015", "11", "12"]
+  end
+
+  test "amz_date/1" do
+    assert amz_date({{2015, 1, 2}, {1, 3, 5}}) == "20150102T010305Z"
+    assert amz_date({{2015, 11, 22}, {11, 31, 51}}) == "20151122T113151Z"
+  end
+
+end
diff --git a/test/lib/ex_aws/utils_test.exs b/test/lib/ex_aws/utils_test.exs
index d74fd845..1532405b 100644
--- a/test/lib/ex_aws/utils_test.exs
+++ b/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