Originally Posted by
anabolism
There does seem to be an element of unclarity. Can the flight flight (or the last sequence of flights) start on the same date as the date of the first flight a year hence, or must it be a day or more earlier than the same date as the date of the first flight?
Well, I can't speak for IATA, but to me there's no lack of clarity. Starting on the same date won't (or shouldn't) be valid.
If your itinerary starts on January 1, 2018, then your itinerary should conclude by December 31, 2018. If you had until the same date in the following year, that would be January 1, 2019, of course. But that's the first day of the next year of your ticket, not the last day of your ticket year.