I look for the timezone automatically changing according to a user location.
I thought this module was doing that.
However, I don't see any effect of the module nor any settings to allow that.
I just found admin/settings/autotimezone
that has an on/off option that is explained to be useless:
"This module can set a session variable named $_SESSION['timezone'] for anonymous users, which is in the form of seconds from GMT. This feature is of no use unless extra code has been added to your site to take advantage of it."

Thanks for help