Bonjour et
Par formule, je ne pense pas que garder en mémoire la valeur de la cellule C1 soit possible.
Par VBA, test ceci
Private Sub Worksheet_Change(ByVal Target As Range)
Dim DateMemoire As Date
If Not Intersect(Target, Range("B1:C1")) Is Nothing Then
Application.EnableEvents = False
Select Case Range("C1")
Case ""
Range("A1") = Format(Range("B1").Value, "ww", vbMonday, vbFirstFourDays)
Case "Livré"
Application.Undo
DateMemoire = Target.Value
Application.Undo
Range("A1") = Format(DateMemoire, "ww", vbMonday, vbFirstFourDays)
Case Else
Range("A1") = Format(Range("C1").Value, "ww", vbMonday, vbFirstFourDays)
End Select
Application.EnableEvents = True
End If
End Sub
A+