Check the time zone setting for this specific day:

1. Go to calendar view
2. Choose the day in question
3. Check the time zone is correct

If the time zone seems correct, then the event may have been created using a different calendar. You can check this by:

1. Hovering over the event label
2. Check which calendar is listed in the description box
3. Check if a different time zone is being used

If this is the case, make sure to change the time zone in the other calendar to match your Microsoft calendar.