[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
this field is a date, so https://docs.python.org/2/library/datetime.html?highlight=datetime.date#datetime.date.today would be more appropriate