Thanks @wroos !
In Collect, the constraint (. > ${date_time_debut} )always throw an error.
I am very surprised to face it so I hope the bug is not in my brain...
Note that the standard XPath type conversions are extended by this specification in the number() function. This extended functionality provides the ability to perform arithmetic with, and compare, date and dateTime strings.
(https://getodk.github.io/xforms-spec/#xpath-operators)
But maybe the number() conversion only covers the day part? You might test it with a manually coded cast, like number(.) and show the result and test number(.) > number( ${date_time_debut} ). It would be interesting how the time part is treated in Collect (and differently in Enketo).
In Collect: number() function and also $-reference in label for dateTime variable seem incomplete. Only if the date part is different, it works as expected.
Hint: date time or datetime (spelling) is automatically converted to type="dateTime" in XML file.
Hint @LN: Type in spec is dateTime, but datetime in ODK template? Same spelling would be better?