Have you checked the man page
? All of the command switches are listed there, so you can see whether one does what you want or not. Having had a quick look through it, though, I doubt you'll have much luck. If I understand correctly, you're asking if you can supply diff with some kind of regular expression which you want it to ignore... IMHO, this is far beyond what diff was meant to do and isn't likely to be supported.
One option that pops to mind is to strip the timestamps off with sed, then pass the modified files to diff. If it discovers any differences, then you know there were differences in the original files, independent of the text in the square brackets.