月初，月末，本月天数的函数

'? firstDayOfMonth("2003-2-4")
'  2003-2-1

'? lastDayOfMonth("2003-2-4")
'  2003-2-28

'? DaysInMonth("2003-2-4")
'  28

Function firstDayOfMonth(myDate As Date) As Date
    firstDayOfMonth = CDate(Format(myDate, "yyyy-m-1"))
End Function

Function lastDayOfMonth(myDate As Date) As Date
    lastDayOfMonth = DateAdd("d", -1, DateAdd("M", 1, firstDayOfMonth(myDate)))
End Function

Function DaysInMonth(myDate As Date) As Integer
    DaysInMonth = DateDiff("d", firstDayOfMonth(myDate), lastDayOfMonth(myDate)) + 1
End Function