[IMP] membership: work on duration, not on date

Task: https://www.odoo.com/web#id=6806&view_type=form&model=project.task&action=333&active_id=131
Pad: https://pad.odoo.com/p/r.yIMRe1VVusFuPpnV
@rim-odoo I think better to use fields.date.today because it directly return formatted value which is expected by ORM