i have: BEGINN ENDING PERIOD Dates
01/01/05 01/31/05 P01-05 07/23/05
01/02/05 28/02/05 P02-05 08/09/05
01/03/05 31/03/05 P03-05 08/08/05
01/04/05 30/04/05 P04-05 08/18/05
01/05/05 31/05/05 P05-05 08/10/05
01/06/05 30/06/05 P06-05 08/12/05
01/07/05 31/07/05 P07-05 08/24/05
01/08/05 31/08/05 P08-05
01/09/05 30/09/05 P09-05
01/10/05 31/20/05 P10-05
01/11/05 30/11/05 P11-05
01/12/05 31/12/05 P12-05
i would like a formula that looking at the "Dates", would place the
date in the correct "Period" according to the "beginning" and "Ending"
of the chart, can anybody help me please?

First clean up the date formats. It appears that the beginning and ending
dates have different day/month orders.

I'm assuming column E is the Dates column.

If the ranges are always the calendar months. You can put
=E2
in cell F2 and copy down, then select column F use:
Format Cells Number Custom Pmm"-"yy

Otherwise, you might want to look into the VLOOKUP function.