Использование Now.Month.ToString("00")
.
Dim str = DateTime.Now.Year & DateTime.Now.Month.ToString("00")
Попробуйте это,
Sub Main()
For i = 1 To 12
Console.WriteLine(GetAutonumber(i))
Next
End Sub
Public Function GetAutonumber(no As Integer) As String
Dim strNum = no.ToString()
Dim str = DateTime.Now.Year & DateTime.Now.Month.ToString("00") & New String("0", 5 - strNum.Length) & strNum
Return str
End Function