Thank you Jeff for your support in your email for this issue, and I will put our discussion here so everybody who has the same issue can learn and everybody can make their comments too.

The issue is that after sync all date data is changed to "?" on both Host and Local side.

I have found a solution in your wiki for this: http://www.seedcode.com/pmwiki/pmwiki.php?n=GoZync3.DateFormats

If I understands it correctly, it was caused by the different date format using on the computer (we use Australia) and the one in GoZync files (United States). The solution is to change the GoZync files to use Australia date format by making a clone of it. But it sounds a bit strange. Does it mean that all the computers and mobile devices have to use the same Date format as the one in GoZync? Our users may use it from all over the world with different date format.

Some helpful reply from Jeff is:

To change the file's date format, make clones of the GoZync files. Then the clones will take on the date format of the computer they're first opened on (Australia). All users need to use the same date format. If FileMaker's "use system formats" is off (a script step), the date format in the mobile FileMaker file doesn't need to match the date format on your mobile users' iPads, so they would only be required to use the Australia date format in the mobile FMGo file, and not in any other of their iPad apps.

Jeff, this is not ideal for our application. We don't want to lock our uses to one particular Date format.

One thing I noticed is that the TimeStamp fields seem to be synced correctly. If I change all Date fields to TimeStamp and save them in TimeStamp format. Then I will only get the Date part of when I view them, would this walk around this problem?

Would this issue affect other types of fields, such as Time and Number?