Can’t tell about other brands, but one reason I decided to pull the trigger and get myself a Wahoo (an Elemnt Roam v2) after a lifetime of not having any cycling head unit whatsoever was that it has a clock (well, and the maps and altitude/inclination info). I’d bet other Wahoos have this feature too. I’d be really, really surpsised if Garmins don’t have this feature - I’d say that stating that “most head units don’t have this feature” is bs.
Granted, I wish the wahoo had a setting to get rid of the “a.m./p.m.” text, which makes the numbers a bit too small compared to other numbers in the screen. When asked support about this they replied me something in the lines of “the unit follows the setting of the phone it is paired to”, so if your phone shows time in 12 hour format, then the unit does so, and if the phone shows time in 24 hour format, so does your unit. Which is a bit nonsensical but it’s still great to be able to see the time of the day there. I wish I could had that thing/could afford such a thing when I had to go to uni and work.
I have a clock on the display on my Garmin 830. I also have sunset displayed on another screen, at I often ride in the evening and don’t want to get caught out after dark.
You might be surprised that keeping time on an embedded system like that adds quite a level of extra hardware to it. And if you actually want good time, then it gets expensive.