One of the users of my Zimbra 7.1.3 installation (on Ubuntu 10.04 amd64) has noticed some strange behavior in the Zimbra web client (both AJAX and Standard HTML versions). When viewing messages in his inbox, if he sorts them with the Received column, they sort based on the time of day but completely ignore the date. For example, all of the messages received at 12:49PM are grouped together at the top of the inbox (a message from 12:49 on 3/16/12 and a messages from 12:49 on 2/1/12 are right next to each other). Folders other than the inbox appear to be sorting correctly.

Viewing the inbox in Thunderbird looks normal - the messages are sorted correctly by date. Other users are reporting no such problem with their inboxes. Any ideas on why this incorrect sorting is happening with the Zimbra web clients, or what else I can do to debug it?