The flight credit expires the date of the original ticket. Essentially, you are rebooking the same ticket - just making a change to it.

Trip credits expire 1 year from when the ticket is cancelled. Basically they are giving you a gift card, which you can use on anybody's ticket.

If you use the trip credit on a new ticket, and then cancel it, the expiration gets extended again (as far as I can determine).

Say you and other family members have trip credits, and then later you buy one ticket for yourself - then you can apply all those trips credits from others. Say then you cancel that trip - then you get a flight credit for that cancelled trip, and a new, single trip credit is issued to you.