A plugin for *any* date, or a made-up calendar?

I’ve been searching but can’t find anything approaching what I need. Here’s what I’m looking for:

I need a plugin that allows me to enter *any* date, ie., any year range I define (the year might be 636, or 1714, etc). Also need a custom calendar (invented month names, and possibly more or less than 12 months in a year). I don’t mind if the time stamp still draws from real system time, but the day/date/year thing must be complete customizable. In the alternative, something that halts the real-time data in the date stamp and lets me enter arbitrary numbers/words there would be great, if that’s the only way to achieve this.

This is for some blogs for a variety of fictional settings, hence the need to remove any realtime here-now date references from the entries and replace with something suited to the fictional setting.

Can anyone point me to such a plugin? Or, if one does not exist,I’d like to talk offlist with someone who could custom-code such a thing.

Thanks,
Teramis