When a calendar event is created with a duration of for example 20
minutes, the end time appears as the start time + 19 minutes.

Because of the rounding of the duration (20 minutes = 0.333... hours),
the end time is incorrectly set (xx:19:59.998...) then displayed. Since
the duration can only be set by the minute, we round it before computing
the end time.

opw-2122853

closes odoo/odoo#40652

X-original-commit: f6ae9f0a
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>