Searching this forum I found:
D = YourDateField ;
startDate = Date ( Month ( D ) ; 1 ; Year ( D ) ) ;
endDate = Date ( Month ( D ) + 1 ; 0 ; Year ( D ) )
5 * Int ( ( EndDate - StartDate ) / 7 ) +Middle ( "0012345501234544012343340123223401211234010012340" ; 1 + 7 * (DayOfWeek ( StartDate ) - 1 ) + DayOfWeek ( EndDate ) ; 1 )
but it doesn't always yield the correct result. Jan, April, June, July, and November show the correct result for the year 2018. Feb, March, May are off by 1. Aug, Sep, Oct are off by 2. Any ideas on why the difference?