Okay, thanks !!
Well I rewrote something more simple :
Sub CTRL20_13_OnClick(eventObj)
Dim ISOdebut
Dim ISOfin
Dim DATEdebut
Dim DATEfin
Set ISOdebut = XDocument.DOM.selectSingleNode(''/my:mesChamps/my:GroupeInformations/my:DatesPrepa/my:PrepaDateReceptFichiers'')
DATEdebut = CDate(ISOdebut.text)
DATEfin = Dateadd(''d'', 20, ...