Tickets purchased now will expire on December 31, 2019, however, according to the Ticket sticky:
"At this time, if never used, all new MYW tickets have an expiration date.
(The date of this expiration will be shown when the ticket purchased.)
However, if this ticket is never used by a guest and allowed to expire,
the
purchase price of the ticket is not "lost" to the guest.
The unused and expired ticket can be turned in at WDW and its original purchase price can be applied to the purchase of a new ticket of like or greater value."
You can see the whole ticket thread here -
https://www.disboards.com/threads/all-about-tickets-upgrades.3570721/
Honestly, if you think there is any chance you may need to cancel, you may want to consider booking a WDW resort package. It will have a far lenient cancellation policy and you won't be stuck with thousands of dollars worth of tickets if you have to postpone your trip indefinitely.