Well you couild use the output of date or time, but if you have the [shell] command you can send a [date( message to it and get the current system clock time and date nicely formatted. Of course you are polling that info so if you are running it using a metronome it could be out by as much as a second. Increasing the metronome to 1/25 second to give you frame accuracy and using [change] to update the seconds field wont work because it's using an external process that has an overhead to run.
Anyway, you will want your output in symbol (string) form to send to symbol boxes or whatever graphical output you use, so symbol to convert any floats.
Here's a couple of examples.
You didn't say you're platform, so I'm assuming MacOSX.
http://www.pdpatchrepo.info/hurleur/datetime.pd