1 files changed, 2 insertions, 1 deletions
diff --git a/packages/gitbook-core/src/shapes/File.js b/packages/gitbook-core/src/shapes/File.js
index cff2219..6c7df3e 100644
--- a/packages/gitbook-core/src/shapes/File.js
+++ b/packages/gitbook-core/src/shapes/File.js
@@ -2,12 +2,13 @@ const React  = require('react');
 const {
     oneOf,
     string,
+    instanceOf,
     shape
 } = React.PropTypes;
 
 
 module.exports = shape({
-    mtime: string.isRequired,
+    mtime: instanceOf(Date).isRequired,
     path:  string.isRequired,
     type:  oneOf(['markdown', 'asciidoc']).isRequired
 });