Date_début = CDate(ComboBox_de)
date_fin = CDate(ComboBox_à)
Sheets("Feuil1").Activate
Set DebCell = ThisWorkbook.Worksheets("Feuil1").Rows("3").Find(What:=Date_début, LookIn:=xlFormulas, lookat:=xlWhole)
Set finCell = ThisWorkbook.Worksheets("Feuil1").Rows("3").Find(What:=date_fin, LookIn:=xlFormulas, lookat:=xlWhole)