[FIX] calendar: correct field type
[FIX] calendar: correct field type

`display_start` was wrongly change to `Date` type during convertion to
new-api.

ygol/odoo,ygol/odoo,dfang/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,dfang/odoo,ygol/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,ygol/odoo,dfang/odoo,ygol/odoo,hip-odoo/odoo