Week numbers and calculating weekly statistics/diagrams

Hello,

I have multiplayer card game in Flash/Perl/C and would
like to add weekly tournaments/player ratings to it.

This means I have to add a table which holds:
player id, weekly score (which I update after each round)
and the week number.

Does anybody has an advice how to save the week number?

If I save it as a timestamp then calculating realtime statistics
(on a player profile click) will probably be CPU-intensive,
because I have to calculate the week numbers each time.

If I save it as string "2010/52" then it's difficult to show
statistics for a period of time (like for the last 12 months)
if there is a new year inbetween.

Maybe there is a better way?

I'm using postgresql-server-8.3.6 and OpenBSD 4.5

Regards
Alex