I am looking for a formula that will calculate the number of Mondays
between 2 given dates (date will always be the 23rd of the month).

I am currently using =INT((D1-WEEKDAY(D1-1)-C1+8)/7)

This is working fine up until it hits a date where the 23rd falls on a
Monday.

e.g. 23rd Sept 23rd Oct 2008 = 4 Mondays (correct)
23rd Oct 23rd Nov 2008 = 4 Mondays (correct)
23rd Nov 23rd Dec 2008 = 5 Mondays (correct)

and so on

But when I get to 23rd Jan to 23rd Feb 2009 (which falls on a Monday),
Excel is returning 5. I am wanting to see 4 at this point, same for
23rd Feb to 23rd March.

How can I amend this formula?
23rd Sept 23rd Oct 2008 = 4 Mondays (correct)
23rd Sept 23rd Oct 2008 = 4 Mondays (correct)