After returning from vacation, my TS-600 telephone for my Talkswitch system wasn’t displaying date and time.  The other extension was working fine, but mine wouldn’t do it.  Tried unplugging the power on the keyset but that didn’t help.
So I finally forced a time update to all the extensions by changing the time in the Talkswitch management program.
That did the trick, date and time now show correctly on all extensions.