I have to disagree with arian487 - I think the only sensible way to implement this 'permanently' would be "past 24 hours" or "past 30 days". I think having it based on calendar day or calendar month will create inconsistency, because there will be a sudden score reset once per day, week, month.
A user might submit his score and see that he is #1 for the day, only to find that the day reset occurred 5 minutes later and his score is gone. I think it only makes sense to use "previous 24 hours"... that way you would always have a queue of a day's worth of high scores to compare yours against, and your score would be guaranteed to have a place in the day's rankings for 24 hours.
*** That said, I think that not having this type of feature is hurting us, and if it's quick to implement that way Riq, PLEASE go for it! I would love to have it.