I would think that the earlier in the month you enter - all other factors being equal - you would have a better chance to rising higher.

I entered my Jazz tune on Feb. 6th, and it's risen to #19 as of today (the 23rd). Certainly someone who entered two days ago would not have a chance to rise that quickly.

Maybe the best solution IS to have a cutoff date of around the 10th or 15th. Anything entered after that would be counted in the following month's ratings.