When payslips are generated from the benefit calendar
view by clicking on the "Generate Payslips" button, the js
puts the start and end date in the context.
Start and end are js Date objects, respectively the start and end of the month

e.g.
start: `Fri Mar 01 2019 00:00:00 GMT+0100 (Central European Standard Time)`
end: `Sun Mar 31 2019 23:59:59 GMT+0200 (Central European Summer Time)`

The Dates are first transformed to a string before being sent to the server
by `time.datetime_to_str`.
Because this function change the timezone to UTC, the start date becomes
the previous day at 23:00:00.
The payslip start date is therefore wrongly set to the previous day.

closes odoo/odoo#31919

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>