okj, i'll try to re-write my previous post without error this time:
Dim d as Date
d=date() '<--- your date here
'last date of month
msgbox DateSerial(Year(d), Month(d) + 1, 1 - 1)
'day number
msgbox Day(DateSerial(Year(d), Month(d) + 1, 1 - 1))

sebastienm