I know there's an accepted answer and that I'm late to the party, however...
Perhaps provide a default date in the function declaration and test for that:
Function NextMondayFromADateOrToday(Optional StartDate As Date = #12/31/1592#) As Date
' objective: if date param not supplied should set StartDate to today
' ??? Problem: with no arg provided StartDate is zero = date value of 1/1/1900
' ??? how to add default to optional paramater of date type?
' ??? or, how to check if an arg was provided if the parameter is date type?
If StartDate = #12/31/1592# Then
StartDate = Date
End If
'If Not (IsDate(StartDate)) Then StartDate = Date
Select Case Weekday(StartDate)
Case 1: NextMondayFromADateOrToday = StartDate + 1
Case 2: NextMondayFromADateOrToday = StartDate + 0
Case 3: NextMondayFromADateOrToday = StartDate + 6
Case 4: NextMondayFromADateOrToday = StartDate + 5
Case 5: NextMondayFromADateOrToday = StartDate + 4
Case 6: NextMondayFromADateOrToday = StartDate + 3
Case 7: NextMondayFromADateOrToday = StartDate + 2
End Select
End Function
If 12/31/1592 isn't sufficiently out of expected range, you could go earlier or later until you start hitting the limits of what a date can handle - a quick test shows Excel 2010 can take date of #1/1/100# and #1/1/9999#