Dim Wbk As Workbook, NomFeui As String, Feui As Worksheet
On Error Resume Next
Set Wbk = Workbooks.Open(Filename:="C:\2017.xlsm")
If Err Then MsgBox "Il n'existe pas de classeur ""C:\2017.xlsm"".", vbCritical, "Vérif": Exit Sub
Err.Clear
NomFeui = Format(Date, "mmmm")
Set Feui = Wbk.Worksheets(NomFeui)
On Error GoTo 0
If Feui Is Nothing Then
Set Feui = Wbk.Worksheets.Add
Feui.Name = NomFeui
End If