diff --git a/shakers.cabal b/shakers.cabal
--- a/shakers.cabal
+++ b/shakers.cabal
@@ -1,5 +1,5 @@
 name:                  shakers
-version:               0.0.21
+version:               0.0.22
 synopsis:              Shake helpers.
 description:           Primitive helpers for Shake files.
 homepage:              https://github.com/swift-nav/shakers
diff --git a/src/Development/Shakers.hs b/src/Development/Shakers.hs
--- a/src/Development/Shakers.hs
+++ b/src/Development/Shakers.hs
@@ -68,7 +68,7 @@
 -- | Unix timestamp.
 --
 timestamp :: Action String
-timestamp = cmdArgs "date" [ "+%s" ]
+timestamp = cmdArgs "date" [ "-u", "+%Y-%m-%dT%H:%M:%SZ" ]
 
 -- | File used for version change detection.
 --