diff --git a/material-ui/material-ui.d.ts b/material-ui/material-ui.d.ts
index 2ef16119c6..afa268ba58 100644
--- a/material-ui/material-ui.d.ts
+++ b/material-ui/material-ui.d.ts
@@ -320,7 +320,7 @@ declare namespace __MaterialUI {
         interface DatePickerProps extends React.Props<DatePicker> {
             autoOk?: boolean;
             defaultDate?: Date;
-            formatDate?: string;
+            formatDate?: (date:Date) => string;
             hintText?: string;
             floatingLabelText?: string;
             hideToolbarYearChange?: boolean;