Option Explicit
Sub Dates_compléter()
Dim c As Range, cc As Range
With Application: .ScreenUpdating = False: .Calculation = xlManual: End With
Set cc = Sheets("Feuil1").Range("b2:b" & Cells(Rows.Count, 2).End(xlUp).Row)
For Each c In cc
If c.Offset(, -1) = "" Then c.Offset(, -1).FormulaR1C1 = "=R[-1]C"
Next
cc.Offset(, -1).Value = cc.Offset(, -1).Value
With Application: .Calculation = xlAutomatic: .ScreenUpdating = True: End With
End Sub