Create 2 users in SOGo.
User 1 has time zone "Europe/Lisbon" (GMT+0) set in https://<sogo>/SOGo/so/<user1>/Preferences#!/general
User 1 has "Day start time": 10:00 and "Day end time": 18:00 and "Show time as busy outside working hours" in https://<sogo>/SOGo/so/<user1>/Preferences#!/calendars
User 1 creates an hour-long event at 13:00 for today.
User 1 shares Personal calendar with any authenticated user "View the Date & Time"
User 2 has time zone "Europe/Warsaw" (GMT +1) set in https://<sogo>/SOGo/so/<user2>/Preferences#!/general
User 2 starts creating an event for today and adds User 1 as a participant in the "Invite attendees" section
User 2 observes that User 1 has a brown "Busy" indicator on the timeline for today for the time 14:00-15:00. This is expected because User 2 has time zone offset +1 compared to User 1.
User 2 observes that User 1 also has a brown "Busy" indicator on the timeline before 10:00 and after 18:00. This is unexpected because User 1 has set that they are busy before 10:00 in time zone GMT+0 so it should be displayed as 11:00 in their time zone GMT +1.
This proves the inconsistency of the display time between normal calendar events (correctly adjusted to the timezone) and the "Busy outside of working hours" (timezone ignored).