It is also useful to note for renewals that it is the date of posting to the account which is important, not the date the flight was taken. However, as I understand it, you do get up until the end of the month in which your renewal becomes due for miles to post.

For example, my Diamond membership year runs from 2 January. If I have enough miles for renewal at Diamond by then then they reset with effect from 2 January; however, if I do not have enough miles they wait until 31 January before renewing at Diamond (if some more come in in that time) or Gold (if not).