I scoured the forums, looking for a chart I
know I've seen somewhere, with no luck. I did find
this thread where
ori shows the code that drives the dates- if I have time later, perhapsibly I'll do the math and put together a rudimentary list.
I believe this is a feature of BUG; however, it's been so long since I played without it that I can't remember. I know one of the sub-mods of BUG lets you set up the clock in certain ways- for instance, I always have mine switch between % of game finished and turn number/total turns in game, I know I couldn't do that before BUG.
EDIT for clarity: Since that last sentence is a but muddled, and I don't have a helpful screenshot, here's an example: Say I'm on turn 75 of a 500-turn game- my clock would alternate between "75/500" and "15%".
EDIT THE SECOND:
Aha! My wanderings have finally borne fruit! Just had to jump a few hundred pages back.

Hope that helps,
Olson.