Hum...
You've reminded me again of the massive holes in the theme language documentation, the manual apparently neglects to mention an entire section on RTC tags in the Theme Tags section, anyway -
here 'ya go.
EDIT: I didn't make it particularly obvious how one would achieve this. There's a few ways it could be done, but I'm pointing out right now that not a single one of them are particularly efficient. Despite what the (dated) CustomWPS page says, and single one of those tags should be able to function as a conditional statement.
But, if you want resolution in minutes, you're probably figuring out just now what you'll need 60 bitmaps for it - or 12 at 5 minutes with blank spaces in the conditional statement...and so on.
You could probably also use the %if or %and tags to check for a specific time, or time range. But its messy.
EDIT 2: I recall once upon a time assisting someone to make an analogue clock, with resolution in hours and 5 minute periods, using layered transparent bitmaps...similar thing, but with a lot less resources required. A digital time/date display using bitmaps is going to be rather a lot more wasteful.
[Saint]