Returning Week # per Month based on Date

Based on the Date/Month/Year, I want to return the week number the date is.


If cell a1 = 1/1/2012 then in b1 I want to return 1 (as it is in the 1st week of the month)
If cell a1 = 2/6/2012 then in b1 I want to return 2 (as it is in the 2nd week of the month)
If cell a1 = 1/31/2012 then in b1 I want to return 5 (as it is the 5th week of the month)
If cell a1 = 2/1/2012 then in b1 I want to return 1 (as it is in the 1st week of the month)

I have tried different versions of WeekNum formula but that isn't working, any ideas?

Any ideas?