Calculate Days Past in a Range of Dates

To get the end of the month based on the date in column A use:

=IF(WEEKDAY(DATE(YEAR(A4),MONTH(A4)+1,1)-1,2)>6,DATE(YEAR(A4),MONTH(A4)+1,1)-3,IF(WEEKDAY(DATE(YEAR(A4),MONTH(A4)+1,1)-1,2)>5,DATE(YEAR(A4),MONTH(A4)+1,1)-2,DATE(YEAR(A4),MONTH(A4)+1,1)-1))

Then to get the number of days between you can either use:

=D4-A4

or:

=NETWORKDAYS(A4,D4)