Challenge Mission Question

Hi guys, I'm just starting to get interested in Challenge missions, but I'm curious how exactly the ladder works. I checked the official site and both wikis but didn't find an answer.

With the Daily best scores, is that a list of the best within the last 24 hours with a rolling 24 hours or is there some magical time when all the scores are wiped out and a fresh 24 hours starts? (midnight PST maybe?)

Same for monthly and quarterly. If I score the best monthly score tomorrow will it be there for 1 month or only until 12/31/08.

Rolling 24 hours, monthly, quarterly, etc.