Before this commit, the date/datetime/daterange fields only allowed for an optional end date field. This meant that the primary date was always the start date. This commit allows the field to do the opposite: with the primary date being the end date, and having a `start_date_field` option for an optional start date. closes odoo/odoo#120695 Signed-off-by: Michaël Mattiello <mcm@odoo.com>