diff --git a/lib/http_signatures/http_signatures.ex b/lib/http_signatures/http_signatures.ex
index af88339..6063ff5 100644
--- a/lib/http_signatures/http_signatures.ex
+++ b/lib/http_signatures/http_signatures.ex
@@ -27,7 +27,7 @@ defmodule HTTPSignatures do
   end
 
   def validate(headers, signature, public_key) do
-    sigstring = build_signing_string(headers, signature["headers"])
+    sigstring = build_signing_string(headers, signature["headers"], signature["created"], signature["expires"])
     Logger.debug("Signature: #{signature["signature"]}")
     Logger.debug("Sigstring: #{sigstring}")
     {:ok, sig} = Base.decode64(signature["signature"])
@@ -71,13 +71,17 @@ defmodule HTTPSignatures do
     end
   end
 
-  def build_signing_string(headers, used_headers) do
+  def build_signing_string(headers, used_headers, created, expires) do
     used_headers
-    |> Enum.map_join("\n", fn header -> "#{header}: #{headers[header]}" end)
+    |> Enum.map_join("\n", fn
+        "(created)" -> "(created): #{created}"
+        "(expires)" -> "(expires): #{expires}"
+        header -> "#{header}: #{headers[header]}"
+       end)
   end
 
   def sign(private_key, key_id, headers) do
-    sigstring = build_signing_string(headers, Enum.sort(Map.keys(headers)))
+    sigstring = build_signing_string(headers, Enum.sort(Map.keys(headers)), headers["(created)"], headers["(expires)"])
 
     signature =
       :public_key.sign(sigstring, :sha256, private_key)
@@ -87,8 +91,11 @@ defmodule HTTPSignatures do
       keyId: key_id,
       algorithm: "rsa-sha256",
       headers: Map.keys(headers) |> Enum.sort() |> Enum.join(" "),
-      signature: signature
+      signature: signature,
+      created: headers["(created)"],
+      expires: headers["(expires)"],
     ]
+    |> Enum.filter(fn {_, v} -> v != nil end)
     |> Enum.map_join(",", fn {k, v} -> "#{k}=\"#{v}\"" end)
   end
 end
diff --git a/test/http_signatures_test.exs b/test/http_signatures_test.exs
index 3dbcdce..b910ede 100644
--- a/test/http_signatures_test.exs
+++ b/test/http_signatures_test.exs
@@ -62,7 +62,7 @@ defmodule HttpSignaturesTest do
 
   test "it contructs a signing string" do
     expected = "date: Thu, 05 Jan 2014 21:31:40 GMT\ncontent-length: 18"
-    assert expected == HTTPSignatures.build_signing_string(@headers, ["date", "content-length"])
+    assert expected == HTTPSignatures.build_signing_string(@headers, ["date", "content-length"], nil, nil)
   end
 
   test "it parses the http signature for a conn" do