Calculate the Pay Period from the date

Hi folks, I'm trying to calculate the pay period for any
date. Right now I can calculate it by taking the
difference from a reference date. However, this limits
me to dates that occur after the reference date. I'm
hoping someone can show me a more general formula that
will work for any date entered. The formula I am using
now is:
=mod(int(B5-37968)/14,26)+1
Were B5 is the date I'm calculating the Pay Period for,
and 37968 is the reference date. The pay periods are two
weeks long, with 26 pay periods in the year.
To make it more interesting the Pay Periods don't start
at the begining of the year. For FY 2004 it started on
12/25/2003, in FY 2005 it will start on 12/24/2004.

Thanks
Mike