You are covered during the dates that are stated in the contract. Let's say your flight departs on the 18th and lands on the 19th covering multiple time zones and you only book insurance until the 18th. You would be covered during departure but you would not be covered at arrival.
Where exactly the coverage ends is complicated. Airplanes usually use UTC during a flight so there is no easy way to determine exactly when the new day starts in airplane time. The most reasonable rule would be that the coverage ends at 11:59pm (on the 18th) in your home time zone (i.e. the same time zone where your coverage originally started). This will be spelled out in your coverage documents and this was indeed the case for my last insurance.
However, insurances are not exactly famous for being reasonable, so it may vary from policy to policy. If in doubt: read the fine print.
If you want to be safe and are worried about a medical emergency on the plane, take the extra day. In my personal experience it makes almost no difference in terms of price: It doesn't affect the trip cost and associated coverage, which is the most expensive part of most travel insurances.
Medical tends to be very cheap to start with and an extra day will make little difference. Looking at my next trip the difference between 6 and 7 days was just $2. If you see a steep increase for a single day, just shop around a little more.