When you create an all-day event in your Outlook calendar and then change the time zone on the computer, the event appears to involve two days

When you create an all-day event in your Outlook calendar and then change the time zone on the computer, the event appears to involve two days.

This behavior occurs because the 24 hours that are assigned to an all-day event (midnight to midnight) are adjusted to the new time zone. If the new time zone is an hour earlier, the event appears to start at 11:00 P.M. on the preceding day and last until 11:00 P.M. of the actual day.

To work around this behavior, change the event's start date and end date to the correct day, and then click to select the All day event check box.

This behavior is by design.