Option Explicit
'H2S : Hour To String
Private Function H2S(HM As Date) As String
H2S = Format(Hour(HM), "00") & ":" & Format(Minute(HM), "00")
End Function
Sub conversion()
Dim chn$, dlg&, lig&: dlg = Cells(Rows.Count, 6).End(3).Row: Application.ScreenUpdating = 0
For lig = 2 To dlg
With Cells(lig, 6)
If .Value <> "" Then chn = H2S(.Value): .NumberFormat = "@": .Value = chn
End With
With Cells(lig, 7)
If .Value <> "" Then chn = H2S(.Value): .NumberFormat = "@": .Value = chn
End With
Next lig
End Sub