Jet Blue's current policy is that the credit must be used up to one year from the date that credit is issued. You can fly beyond that date, just have to purchase a ticket with the credit before the year is up.
I just did this for a trip, so I know this is still the policy. Other airlines want you to FLY by the one year date from that date of purchase, Jet Blue (so far) does not. You can fly in September, just have to use the credit to purchase your ticket before the credit expires.
Keep in mind that if your new flight goes down and you get another credit, that credit is backdated to the original one you used and carries that old expiration date. For example: Buy a ticket now, it goes down next month and you are issued a credit. Goes down again two months from now, issued another credit. If these credits are in your True Blue account it's a total accumulated credit balance that can be used toward purchasing a new ticket, If NOT in a True Blue account, only one of these credits can be used toward another single ticket purchase as they are assigned a different file number each time a credit is issued. Only one credit can be applied to one ticket purchase, just like the Disney park pass rules for upgrading.
Let's say both credits are in your True Blue account and you use this next July toward a ticket for September travel. The September trip price goes down. Your new credit will still have the expiration date of the oldest credit you used toward this purchase even if those credits did not cover the entire September purchase.
Apologize for the wordy explaination!
