A family of Microsoft word processing software products for creating web, email, and print documents.
Hi,
Thank you for replying.
Instead of sharing the file through the SharePoint, try sending the email across all the reviewers and check if the time stamp works normally.
If this works as expected, then the issue would be something to do with the permission when working in the SharePoint.
In this case you may post the question in the following link for more help on this issue:
http://social.technet.microsoft.com/Forums/en-US/home?forum=word
Let us know the results to help you better.