Updated timestamp to work in regions with an UTC offset

src/wp8/ExtensionMethods.cs

@@ -35,6 +35,7 @@ namespace Cordova.Extension.Commands
         public static long ToJavaScriptMilliseconds(this DateTime dt)
         {
             return (long)dt
+                .ToUniversalTime()
                 .Subtract(new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc))
                 .TotalMilliseconds; 
         }