The desired anonymous nature of the genre challenge would seem to eliminate much older images...and, I have tried to stick to "do it that month" and in that found the real challenge.
Sure, some months I simply could not get it together to get an entry and on others did do that and made a real effort to get a shot.

I like staying with time limits, perhaps announce the challenge genre a couple of weeks earlier but don't open the entry page until the month arrives...who knows.  But, time limits are part of the challenge and not only the subject of the genre.