Probably, yes. I mean, all you need is to know how to fetch the date from the system with Python, and you can write a clock 'style' that's basically the same as one of the other clock styles, but fits the date in there.

Actually, this also means you can stick in fancy things... Like, make a 'clock' style where the top half is the time, and the bottom half is, say, Internet data traffic, wifi connection quality, and cpu + mem usage (if python can display all those things without being too processor intensive... As far as I know it should be able to). Hmmm... I need to get on that python learning thing I've been meaning to do.

- Edit -

Just keep in mind that you have space constraints - the N900's status area clock icon area can push everything else in the status-bar to the left, which means if you make your hypothetical clock style too wide, you'll make it impossible to press on the actual status-bar's area that a running application uses to possibly put its menus into, or whatever. Especially in portrait. Not a big deal, just something to be aware of for any python-savvy would-be clock style makers.