The clock on the menu bar keeps stopping since loaded mountain lion, why?

I'm not sure if this is a cure but they won't hurt:

SMC RESET

http://support.apple.com/kb/HT3964

• Shut down the computer.
• Unplug the computer's power cord and ALL peripherals.
• Wait 15 seconds.
• Attach the computers power cable.
• Wait another 5 seconds and press the power button to turn on the computer.

The 5 second timing is important to a successful reset.

PRAM RESET

Zap the PRAM

• Shut down the computer.
• Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously in step 4.
• Turn on the computer.
• Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
• Hold the keys down until the computer restarts and you hear the startup sound for the second time.
• Release the keys.

Select  ▹ System Preferences ▹ Language & Text ▹ Format ▹ Region: your country.

I also have this problem since the most recent updates (applied about a week ago) on both my iMac and MacBook Pro. It'll work for a while and then stop. If one selects the time on the menu bar it updates but doesn't continue to update every second. Timestamps on files appear to be correct and it updates every second in the date/time preferences panel.

My region is set correctly; but I'll try the SMC & PRAM reset; but I think the time itself is fine; just the update on the menu bar that doesn't work

Well, I restarted both Macs after my post above (without the SMC or PRAM resets) and all seemed well for a few days. Then the iMac stopped updating (MB Pro fine) and I also noted that when Time Machine kicked in the menu bar icon didn't animate as usual so it appears that the problem extends to all menu bar items. Then today, my trial licence for iStat Menus 4 expired. I purchased a new one and updated to 4.03 and suddenly the time is updating on my iMac menu bar again. So, if you have iStat installed I suggest either uninstalling iStat or buying a licence and upgrading as I suspect there may be a conflict with the last Apple updates.

It really seems as if iStat Menus has something to do with that... after I unistalled it, the clock updated immediately. Thanks for your tip Berlin Canary!

Happy to help stuggi! Thought these sort of conflicts were a thing of the past; but I guess menu bar plug-ins are still capable!