Changing Information In If Formula?

I have a timesheet worksheet that a previous employee created and I need to alter a formula. Unfortunately, I can no longer contact them.

Anyways, I need to alter an "if" formula that is associated with a drop down list. As of now, the drop down list is used for a 2 week period ending on a specific date. When the date is selected, the calendar automatically aligns the proper dates to the days of the week. I believe it is able to do this with an "if" formula that is used by the numbered days of the month. For example; if I select January 14th, 2012 as the 2 week period ending date,

On the 14th the "if" formula shows as this:
=IF(ISBLANK($P$2)," ",$P$2+1-1)
On the 13th, it shows as this:
=IF(ISBLANK($P$2)," ",$P$2-1)

I noticed a trend here with the last digits 2+1-1 and 2-1, so i attempted to go with this trend and add numbers as the dates went on, but it did not work.

I need to change this so that instead of a 2 week ending period, it is for a full month. So, if I were to select January 2012 or any month, all the days of the month would align correctly with their days of the week.