I agree the term "12 months" is vague. AA also insisted that the last flight (not just the last sequence of flights) must depart the day before the one year mark. Interestingly, the AA RTW desk was happy to have the last flight depart on the one-year mark, but ticketing refused to issue it.

I am currently on a rtw issued originally by BA in South Africa. Although throughout process of the original issuing and subsequent reissuing of the ticket I usually spoke on the SA telephone line, I was never sure of whom I was talking to or where he or she may have been physically located. I also used the Brazilian 0800 number, which often connects through to people in Portugal. In any case the first flight was on 27 April 2017 and the last flight is currently scheduled for 27 April 2018. The whole issue of 1 year or 365 day limit never came up.

I can understand that. Count one year from, say, January 1, 2018. The year ends on December 31, 2018. January 1, 2019 is the first day of the next year.

I don't think that "12 months" is vague at all. It's the same as my example immediately above. January 1, 2019 is the first day of the 13th month, not the last day of the twelfth.

And I apply the same logic to 365 days. That's even easier! Use Excel to put a date in and add 364 to it. (If you add 365, you will be at the 366th day.)

Right, and the AA RTW desk was happy to have the last flight depart per that guideline (that is, one day before the first flight the previous calendar year) but AA ticketing insisted it be moved one day before that. That's why I said one day before the one-year mark.

Well, I find the term vague because a "month" means different things in different contexts. It's often used as a standardized 30-day interval, other times a calendar month (which might be 28, 29, 30, or 31 days).

Yes, "365 days" is clear and not ambiguous, except when a leap year comes into play. If my first flight is, say, February 28, 2016 (a leap year that has February 29), can my last flight be February 27 of the following year, or must it be February 26?

If you call JAL ticketing/reservation office in JAPAN, you can fly on February 28, 2017.

I suggest that it's clear and unambiguous regardless of whether or not it's a leap year. 365 days is 365 days. Doesn't matter when the month name or year number changes, or how many days there are in a year.

Actually I meant 366 days if the date range crosses a February 29th (in the present year or otherwise). Just couldn’t be bothered to write it. :p

You have until one day preceding the date one year hence of your first coupon to commence your final flight.

Do the rules provide for leap years?

You can get some idea about meaning of 'month' in airline ticket validity from CX online reservation.

HND-HKG 15Oct
HKG-HND 15Nov
-> 1 month, V class fare applied (V class fare is defined as one month. not 30days/31days/...)

HND-HKG 15Nov
HKG-HND 15Dec
-> 1 month, V class fare applied

HND-HKG 15Nov
HKG-HND 16Dec
->1 month + 1 day, V class fare can't be applied.

HND-HKG 12Nov
HKG-HND 26Nov
-> 14 days, S class fare applied.

I agree. I think this is where the confusion arises. If you buy a ticket that is valid for 1 month and depart on 01MAY you can return on a flight that departs on 01JUN but arrives on 02JUN. See example below, the QL*55ZN1 fare is valid for 1M and the NVA for the return sector is 01JUN, even though the flight can arrive on 02JUN.

They don’t need to. Ticket validity is governed by IATA rules. Fares can specify a shorter validity than one year, but it is unusual.

Right, but do the IATA rules say one year, 12 months, or 365 days? Or Stardate, perhaps?

See IATA Resolution 735.

I shall try to dig out the exact wording if you are particularly interested.

I am fairly sure it is "twelve months", i.e., the same date next year.

OK, so I'll ask the obvious question - what about a ticket starting on 29th February? :D

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?