Wow there is all kinds of stupid going on…the event has 21 more days to complete, the reward has a 1D+ timer to be claimed.. its really not that hard.
The event will last 21 days, and you can earn rewards during that 21 day period.
However, rewards can acquire a separate 'claim' timer once they've been earned and are claimable. Sometimes that's longer, sometimes it's shorter.
If you wait 1 day and 7 hours, then you'll find out for sure; but personally, I'd just go ahead and claim them.
The event will stay on for 21 days, you can claim form thier. It will be gone from stash but stays in event tab.
Works same for all these type of events, for example the BG milestones.
Timer in STASH is set as a limited time, but for EVENTS that go longer the rewards are always still claimable from EVENT tab for the duration of the Event.
See mine below (Ally BG Event), Milestones prior to 7.5m have already dropped off the STASH availability, but you can see they are STILL AVAILABLE on the actual EVENTS screen.