First and Last day of current month

=DateSerial(Year(Now), Month(Now), 1) for first day of the month 
=DateSerial(Year(Now), Month(Now)+1, 0) for the last day of the month.

--previous month last
=DateSerial(Year(Now()), Month(Now()), "1").AddDays(-1)

--previous month first
=DateSerial(Year(Now()), Month(Now()), "1").AddMonths(-1)