VBA increment date error


I've never done VBA before but a friend has asked for help so I've been working on his project. I'm trying to increment a given date by one day but I'm getting an "Object Required" error in the DateAdd function. As far as I can tell I'm putting in an object (firstDate)...

The date entered,for my testing purposes at least, has been 12/03/2012 in that format. The Format and CDate functions seem to work fine and parse the date.

Here's the code, I get the error at the line with DateAdd in it.

Sub GetDate()

Dim strDate As String

strDate = InputBox(Prompt:="Enter the first day of the week in dd/mm/yyyy format.", _
      title:="First day of the week", Default:="")

    If strDate = "" Or strDate = vbNullString Then
        Exit Sub
        Dim firstDate As Date
        firstDate = CDate(Format(strDate, "Short Date"))

        'Add to O and 6 after
        For inc = 0 To 6 Step 1
            Range(Chr(79 + inc) & 1) = firstDate
            firstDate = DateAdd(DateInterval.Day, 1, firstDate)

    End If
End Sub

Appreciate any help.

Very well...

DateAdd("d", 1, firstDate)

But: I'm not that fussed if anyone reposts a comment of mine as an answer. I do read the comments as well as the answers. Maybe because I have this habit of suggesting answers in comments...