Adding a system time readout is so trivial to do, both as a mod (ZScript only) but also not much work at all to add natively in-engine to the automap. I'll probably take a look at it some time (optional feature that's initially turned off, of course)
(I don't understand why this thread has 4 pages worth of posts for something that essentially could be added into the engine with 10 minutes worth of coding LMAO)
(Also holy crap, this thread is 19 years old. I might win an award for "longest time taken for a feature request to be fulfilled" award here ;))
There's an alt-hud option to show system time on the top right corner of the screen.
Despite this though, it'd be nice to also see it on the automap, but so all bases are covered ¯\_(ツ)_/¯