Yeah, if CoS or user time zone is wrong, then all events will be off in all HTML/mobile views.
But even with the viewing user's time zone correct, there's still a bug displaying individual events created by ZCO, which puts the local time zone in the DESCRIPTION but uses UTC time for DTSTART/DTEND.
http://bugzilla.zimbra.com/attachment.cgi?id=9062 shows the "Fax Server Group" meeting time correct in the day view on the left, but the meeting detail view on the right has an incorrect Date: header.