In all likelyhood, it's choking on your date command. Check to see if you have any new directories with odd names.
Instead of calling an outside program, how about using localtime() or one of the Date modules?

Update: Despite tye probably caqlling it correctly, I'll stand by my suggestions. You might want to call the full path to the date program if you insist on an outside call. After all, how can you be sure you'll get the right one?