Its completed by. I'm always confused whether this means land by date or final segment must depart by (stupid overnight & redeye). Heres a extreme example that may help (well helps me explain better it since it's real world to me)


Alaska

1/25/2013 I booked a flight departing Boston on 7/23/13. I arrived Anchorage 7/24/13. My stop-over has me continuing to Barrow on 6/1/14 (Not a typo, 1 year stop-over). According to the Cancel flight screen it says "You may use the value toward travel completed by July 23, 2014" Thats exactly 1 year from departure on my first segment.

In your example on Alaska i'm not 100% sure how it works as its round trip (its been years since i booked on Alaska as Round trip). Whether its completed by one year from first segment (11/1/14) or 1 completed by year from the return (11/30/14). I believe, the whole thing including the return flights have to be completed by 11/1/14 like with my stop-over ticket where it ignores the date of my ununsed segment


Southwest

With Southwest if you bought 1/1/14 you would need to complete travel by 1/1/15 or your credit is gone. Technically Southwest will extend the credit for a $100 fee.

If you book on Southwest when the schedule opens and yiu have to cancel on day of departure, you could find yourself with only 4-6 months to use the credit as the 1 year clock started ticking the day you hit purchase.