"Mark all read" resetting the times of the last post

I've had this happen a couple of times. I click "Mark all read" and the posts that were unread, I think, have their times reset.

I have no idea whats causing this.

The clocks of the two servers are out of sync if someone makes a post on server a which is running 15 minutes behind server b and you are on server a it won't mark read until it passes server b's time.

I've installed ntp on both servers so the times should be synchronized now.