Perhaps they all have a good sense of time, plenty of people don't wear watches, might not look at their mobile phone and still arrive when they are supposed to meet.
In the years before mobile phones, I've gone years without wearing a watch, my estimation of the time being in error of the actual time rarely over 10 minutes, even after several hours had elapsed. Perhaps it is a skill one can develop.
However, considering that the watch function has already been seamlessly integrated into mobile phones in our time, it would be very hard to imagine that such basic functions have not been integrated into their everyday equipment in the 24th century. For all we know, their commbadges automatically register it when an appointment is made ('see you at 1500'), and issue a warning 15 minutes in advance. Or some other (integrated) PDA everyone carries but which we've just never seen onscreen.