if you want to understand if the data is date  format, you can use isdate()
function.
Something like;

If isdate(Range("A1").value)=true then
'The Codes
End if

and if you want to change the date to the next month.I prefer this example
which i copy from "MS Visual Basic Help"

Dim FirstDate As Date ' Declare variables.
Dim IntervalType As String
Dim Number As Integer
Dim Msg
IntervalType = "m" ' "m" specifies months as interval.
FirstDate = InputBox("Enter a date")
Number = InputBox("Enter number of months to add")
Msg = "New date: " & DateAdd(IntervalType, Number, FirstDate)
MsgBox Msg