Bon et bien mon trigger marche enfin !
Pour rappel : le format datetime : c'est affreux à utiliser...
create trigger tg_modif on sysdba.matable
for update
AS
DECLARE
@machinid standardid,
@accountid standardid,
@description varchar(64),
@userid standardid,
@modifydate datetime
select @activityid = activityid, @accountid = accountid, @description = description, @userid = userid, @modifydate = CONVERT(char(24), CONVERT(datetime, getdate(), 103), 103) from inserted
where description like 'Moi'
BEGIN
insert into actmodif values ( @activityid, @accountid, @description, @userid, @modifydate)
END