Function module for particular dates have within period and samefiscal year

Hi Experts,

My requirment:

selection screen i given posting date to and posting date from as a select-options.

How to those dates have within a particluar period and same fiscal year.( see example jan 1st to 28th jan period = 1, 29th jan to feb 25 = period 2.)

i think that one function module is there.

tell me function module name.

Thanks

jaya