The first day of the previous month is always 1, to get the last day of the previous month, use 0 with DateSerial:
''Today is 20/03/2013 in dd/mm/yyyy
DateSerial(Year(Date),Month(Date),0) = 28/02/2013
DateSerial(Year(Date),1,0) = 31/12/2012
You can get the first day from the above like so:
LastDay = DateSerial(Year(Date),Month(Date),0)
FirstDay = LastDay-Day(LastDay)+1
See also: How to caculate last business day of month in VBScript
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