
The event passed to formatEventTime() is reused at a later point.
Therefore, we are not allowed to manipulate the event directly.
This fixes an issue where the user clicks on a multi-day all-day event
the second time before the event gets garbage collected and the event
then is one day shorter.
Fixes 528ee01fef
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2184>