Appreciate advice in advance..

I have set calendar dates equating to a Reporting Month ie

01/01/06 - 31/01/06 = M01
01/02/06 - 28/02/06 = M02

now if I have a cell that relates to when a contract closes (eg 15/01/06)
how can I set the formula in next cell to equal the "M01"

Will need to run this out for the whole year, so not sure I can build that
many < nested IF functions

Thanks