Sub Conversion()
Dim a, b, tablo, resu(), i&, x$, j%
a = Array("Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")
b = Array("Janv", "Fév", "Mars", "Avr", "Mai", "Juin", "Juil", "Août", "Sept", "Oct", "Nov", "Déc")
With [A1].CurrentRegion
    tablo = .Resize(, 2) 'matrice, plus rapide, au moins 2 éléments
    ReDim resu(1 To UBound(tablo), 1 To 1)
    For i = 1 To UBound(tablo)
        x = tablo(i, 1)
        For j = 0 To 11
            x = Replace(x, a(j), b(j))
            If IsDate(x) Then resu(i, 1) = CDbl(CDate(x)): Exit For
    Next j, i
    .Columns(2).NumberFormat = "mm/dd/yyyy"
    .Columns(2) = resu
End With
End Sub