Select a single day in the month view; type a new entry;

Quit evolution [ well ok, it crashed ], re-load the calendar:

Notice the appointment is now spanning two days; labeled 1:00 to 1:00 the
subsequent day. Clicking on the appointment reveals that - in fact - it is
a single-day appointment etc.

No time-zone settings, and/or local-time settings changed between entering
this appointment and it's movement. Indeed - the timezone is Europe/London
for both the system and Evolution.