or does it do a rolling update per X amount of time? Say... the last 30 days.

i think the last 90 perhaps

You've seen the code for this site?

I was wondering if it was something simple :)

What language is that btw?  (I'm sure another dumb question...)

Yes.

http://www.chess.com/forum/view/help-support/average-time-per-move2

It's based on all of your games.  Because of this, it is not a meaningful statistic.  As an example, I have played over 1000 games.  As a result, regardless of the time that I take for a move, my time per move changes little.

I've had opponents who have a low time per move, but they now take 1 or 2 days to make their move, but their time per move stays low.

So it does take all games into account. I believe it should be all games in the last... 30, 60, 90 days... etc...

Since the time-per-move has no effect on the time limits of any particular game, and as a statistic is of questionable value, those who pay any attention to it and use it as a factor in their decisions deserve any unfavorable outcomes which result.

In other words, screw 'em.

It is fristrating that it effects entries into tournaments though

A little harsh, IMO. Yes, in any particular game, it doesn't matter. But, some may be restricted from joining tournaments that are supposed to be quick.

The TD may have set this meaningless statistic at a low number, unaware that it includes every game since joining.