Number of specific days in a month

[abailey3]

I'm working on building a new invoicing system. One of our billable items is an update that runs every Thursday. We bill on a monthly basis. What's the best way of calculating the number of Thursdays in a specified month?

As a part of some other calculations used in this model, I do have specific month (i.e. "August", "September") and year (i.e. "2010", "2011") fields. Piggybacking those would be great.

[comment]

See if this helps:

http://fmforums.com/forum/topic/42318-week-number-of-month/page__p__197309#entry197309