[FIX] calendar: all day event shouldn't store in UTC

With a user in a distant timezone, like UTC +/- 11

Create a all day event.

Before this commit, the event was displayed across the wrong days
This was because the dates were converted from the user's timezone
into UTC before being stored.

Which is:
- start date at december, 12
- if the user's tz is utc-11: the date becomes december, 11 (at 1:00pm)
- at rendering, it becomes december, 11 (at 7:00am)

After this commit, we store the datetime field "as if" it were a date field
that is, we don't convert it to any particular tz

It is worth noting that, if one removes the allday mapping in the xml view,
an event that has been created as allday will get its start and stop date converted
to the browser's timezone, and will NOT be displayed as from 8am to 8pm as one could expect
This is considered as a limitation of the code, that was present before this commit anyway.

In any case, please do not revert this commit :D

OPW 1891110

closes odoo/odoo#27949

2 files changed, 23 insertions, 6 deletions

diff --git a/addons/calendar/models/calendar.py b/addons/calendar/models/calendar.py
index 25377659563..8ec5dcb195c 100644
--- a/addons/calendar/models/calendar.py
+++ b/addons/calendar/models/calendar.py
@@ -890,18 +890,18 @@ class Meeting(models.Model):
     def _inverse_dates(self):
         for meeting in self:
             if meeting.allday:
-                tz = pytz.timezone(self.env.user.tz) if self.env.user.tz else pytz.utc
 
+                # Convention break:
+                # stop and start are NOT in UTC in allday event
+                # in this case, they actually represent a date
+                # i.e. Christmas is on 25/12 for everyone
+                # even if people don't celebrate it simultaneously
                 enddate = fields.Datetime.from_string(meeting.stop_date)
-                enddate = tz.localize(enddate)
                 enddate = enddate.replace(hour=18)
-                enddate = enddate.astimezone(pytz.utc)
                 meeting.stop = fields.Datetime.to_string(enddate)
 
                 startdate = fields.Datetime.from_string(meeting.start_date)
-                startdate = tz.localize(startdate)  # Add "+hh:mm" timezone
-                startdate = startdate.replace(hour=8)  # Set 8 AM in localtime
-                startdate = startdate.astimezone(pytz.utc)  # Convert to UTC
+                startdate = startdate.replace(hour=8)  # Set 8 AM
                 meeting.start = fields.Datetime.to_string(startdate)
             else:
                 meeting.write({'start': meeting.start_datetime,
diff --git a/addons/calendar/tests/test_calendar.py b/addons/calendar/tests/test_calendar.py
index 12547c8f695..e04866f1f68 100644
--- a/addons/calendar/tests/test_calendar.py
+++ b/addons/calendar/tests/test_calendar.py
@@ -275,3 +275,20 @@ class TestCalendar(TransactionCase):
         self.assertEqual(test_event.res_model, test_record._name)
         self.assertEqual(test_event.res_id, test_record.id)
         self.assertEqual(len(test_record.activity_ids), 1)
+
+    def test_event_allday(self):
+        self.env.user.tz = 'Pacific/Honolulu'
+
+        event = self.CalendarEvent.create({
+            'name': 'All Day',
+            'start': "2018-10-16 00:00:00",
+            'start_date': "2018-10-16",
+            'start_datetime': False,
+            'stop': "2018-10-18 00:00:00",
+            'stop_date': "2018-10-18",
+            'stop_datetime': False,
+            'allday': True,
+        })
+
+        self.assertEqual(event.start, '2018-10-16 08:00:00')
+        self.assertEqual(event.stop, '2018-10-18 18:00:00')