Sorry if this has come up before. Didn't find anything in search except the all day event timezone bug.
This is driving me nuts. It seems to be related to some PCs rather than user login or browser version or type.
A user will put a new entry in and have it appear in their calendar 17 hours ahead of the time they selected. Tried it on this PC with Firefox 126.96.36.199 and IE 6.
If I log onto the same user's calendar on my PC (Ubuntu with FF 1.07) it works fine.
I have other users with FF on win XP where it seems to be working fine.
The time displayed in the Zimbra UI matches the system time.
Zimbra version Version 3.1.0_GA_279.DEBIAN3.1 Mar 31, 2006