A-a-a-a-and merry birthday.
DecimalClockPolRail.tar

Enjoy. Seems to support everything all the stock clocks support (so, everything except 12-hour mode and image icons at the same time, which none of the stock clock styles work with anyway).

Really sorry I promised this like a month ago and never got around to it.

- Edit -

Sorry, in case you don't know how to use tar:

You'll need root access (preferably with rootsh, but other ways are possible), then type this into an X-Terminal (this tutorial assumes you have rootsh installed):

Elevates that x-terminal to root user.
Changes Directory to "/", the root directory (This is because when I .tar these clockstyles, I tar them from /, so they have the entire objective path in them - so if you untar them from /, they go where they belong)
Replace "[path/to ... attached]" with where-ever you put the .tar file. If you downloaded it to the part of your N900 that you see through USB Storage Mode on your computer, the path is /home/user/MyDocs/ - so if you, say, have a folder called Downloads in there, and you put the above file in there, it would be at "/home/user/MyDocs/Downloads/DecimalClockPolRail.tar". Feel free to ask if you can't figure out what the path is to where you saved the file.
This restarts the status menu, so Advanced Clock Plugin can reload all the clocks. So if you copied the file to it right, it'll now be able to display the clock style you unpacked. So now go to the settings menu for Advanced Clock Plugin and select "DecimalClockPolRail". (Alternatively you can select it first, then killall hildon-status-menu, but it won't show up until you do both of these).

You can then type "exit" in X-Term. This just exits you out of the root shell. You can type exit again to exit x-term as a whole. You can just close it and skip typing exit both times, but if you use "exit", the commands you used will be available in the history the next time you open x-terminal.