Make timestamp 64-bit

Change-Id: I5d0f14a0bfda5cfc5e483836bb63e76b3334b95d
diff --git a/gatekeeper.cpp b/gatekeeper.cpp
index cae98e8..b8e51e7 100644
--- a/gatekeeper.cpp
+++ b/gatekeeper.cpp
@@ -140,7 +140,7 @@
 }
 
 void GateKeeper::MintAuthToken(UniquePtr<uint8_t> *auth_token, uint32_t *length,
-        uint32_t timestamp, secure_id_t user_id, secure_id_t authenticator_id,
+        uint64_t timestamp, secure_id_t user_id, secure_id_t authenticator_id,
         uint64_t challenge) {
     if (auth_token == NULL) return;
 
diff --git a/include/gatekeeper/gatekeeper.h b/include/gatekeeper/gatekeeper.h
index 9d94fc2..9fb9661 100644
--- a/include/gatekeeper/gatekeeper.h
+++ b/include/gatekeeper/gatekeeper.h
@@ -135,7 +135,7 @@
      * The format is consistent with that of hw_auth_token_t.
      * Also returns the length in length if it is not null.
      */
-    void MintAuthToken(UniquePtr<uint8_t> *auth_token, uint32_t *length, uint32_t timestamp,
+    void MintAuthToken(UniquePtr<uint8_t> *auth_token, uint32_t *length, uint64_t timestamp,
             secure_id_t user_id, secure_id_t authenticator_id, uint64_t challenge);
 
     /**