There is an app (not in the app drawer) called badger that updates the date on the calendar icon on the stock launcher. If you've seen any error messages about badger, that may explain why it is wrong.
I would think that actually opening the calendar app might force it to update if you're not seeing error messages. Or put in a fake event that repeats every day at 12:01 am (particularly useful if you have the phone on silent overnight), as I believe having the app notify you also forces that icon to be re-badged.
(Just as an explanation, as far as I know there is no mechanism in android to dynamically change app icons, so Motorola used the badger app kludge to change the calendar icon, as well as add unread badges to the gmail, email, messages, and phone icons, but only when you use the stock launcher.)
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