calendar: Start ranges at 0:00 and iterate in whole days

calendar: Start ranges at 0:00 and iterate in whole days

Using a starting time other than 0:00 will prevent events before the
chosen starting time from showing up for that range. This was causing
events before 12:00 to be missing in the shell calendar on the first day
of a range.

Fix this by always starting at 0:00 and then incrementing by days rather
than a time value that depending on DST or leap seconds may or may not
correspond to a day.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2023>