I don't think it's one year from purchase, I think it's one year from outbound. For example, for an H fare, the "Max Stay" portion of the rules say,
FARE RULES TEXT TRAVEL FROM LAST STOPOVER MUST COMMENCE NO LATER THAN 12 MONTHS AFTER DEPARTURE FROM FARE ORIGIN.
I think I need to call to change within a year from ticketing--but I have to do that anyway, since I would need to change it before my initially purchased return date (which can only be booked as far out as Feb 2018).