Force file name format

You can use use GetSaveAsFilename to get the saveAs filename then test the result before saving

This will not give you exactly what you want
Testing for mmdd would take more coding

Sub FileSaveAs()
Dim sFile As String
Dim sFname As String
Dim bFnameTestFail As Boolean

GetFileName:
'get path & filename
sFile = Application.GetSaveAsFilename(fileFilter:="Excel Files (*.xls), *.xls")

'test for cancel button selection
If sFile = "False" Then
   Exit Sub
End If

'test file name
If Not Len(sFname) = 13 Then
   MsgBox "Incorect file name format"
End If
If Not IsNumeric(Left(sFname, 4)) Then
   MsgBox "Incorect file name format"
End If
If Not Mid(sFname, 5, 1) = "-" Then
   MsgBox "Incorect file name format"
End If
If Not IsNumeric(Mid(sFname, 6, 4)) Then
   MsgBox "Incorect file name format"
End If

If bFnameTestFail = True Then
      MsgBox "Incorect file name format"
      GoTo GetFileName
End If
ActiveWorkbook.SaveAs Filename:=sFile
End Sub