通过javascript从移动应用程序将正确的日期传递到服务器

I am trying to create a mobile app using PhoneGap. I need app to work in disconnected mode. So I am storing data locally and when connection becomes online I sync it with remote data. To perform remote sync I need to pass lastmodified date to the server so it knows which updates to push to the client. The problem is that server time and client times are different. The point of truth is server date, so I need to be able to interpret client date somehow on the server to be in-line with server date or pass server mathcing date from a client. Any suggestions on how to do it? I was thinking of using I am trying to create a mobile app using PhoneG