convert your string to dateTime type and than AddMonths, below is my example:
dim strDate as string="16/04/1987"
dim dtDate as DateTime
If DateTime.TryParseExact(strDate, "dd/MM/yyyy", Nothing, DateTimeStyles.None, dtDate) Then
console.write("Date Add Month::" & dtDate.AddMonth(1))
else
console.write("invalid format")
End If