Invoice Number = Now.Year & Now.Month & ?????

Dim str = DateTime.Now.Year & DateTime.Now.Month.ToString("00")

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