Originally Posted by
Dr. HFH
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.
But as I (and others) pointed out, a 1M ticket can start on 01APR and ends with a flight departing on 01MAY and arriving on perhaps 03MAY. This is where we see an inconsistency.