ISDATEFIRST

Returns whether a date is the first of a week, month or year.

REMARKS

ISDATEFIRST_OFAWEEK

Returns whether a date is the first day of the week.

ISDATEFIRST_OFAMONTH

Returns whether a date is the first day of the month.

Public Function ISDATEFIRST_OFAMONTH( _ 
Optional ByVal dtDateValue As Date) As Boolean 

Dim iDay As Integer 
    Application.Volatile 
    
    If VBA.IsMissing(dtDateValue) Then 
        dtDateValue = VBA.Date 
    End If 
    
    iDay = VBA.Day(dtDateValue) 
    If (iDay = 1) Then ISDATEFIRST_OFAMONTH = True 
    If (iDay > 1) Then ISDATEFIRST_OFAMONTH = False 
End Function 

ISDATEFIRST_OFAYEAR

Returns whether a date is the first day of a year.

For instructions on how to add these functions to a workbook refer to the page under Inserting Functions