It's a bit of a workaround but you can use teh EOMonth function - EOMonth gives the serial number of the last day in the same month (providing arg 2 = 0)
EOMonth("01/01/01",0) retuens the serial number for 31/01/01
therefore =(EOMonth(serialnumberfor1stofmonth,0)-serialnumberfor1stofmonth)+1 ('cos you're starting on day 1)
will give the number of days in a month
HTH
Geoff