What's my arrival time?
Good question! +1 day means that your flight arrives at your destination the day after you departed. However, that does not mean that your flight will take a whole full day, only that the next day has begun when your flight is scheduled to land.
A Boston to Iceland flight is marked +1 day on the 1st of March 2019
The flight departs from BOS at 06:20 pm and arrives in KEF at 4:50 am local time.
Actual flight time is only 5h and 30m, and obviously, you'll land in Iceland at 11:50 pm on the 1st of March, Boston time (PDT). However, due to the time difference between Boston and Iceland, the clock in Iceland when you land will be 4:50 am (GMT) local time on the 2nd of March, the next day. Hence the +1 day.
This is a universal rule in the flight industry and the reason why flights are sometimes marked +1 day.