Others said it first, but time zones, Steam updating and other stuff all make it so that judging how many days exactly you have to finish events like this is iffy. An estimation from the devs (with timezones) would certainly be appreciated, even if I didn’t personally have a problem.
Meanwhile, if you’re online at least at some time during the event, you can check the event quest timer in the… lower right corner of the quest window, I think. It’s probably the most precise info we can get, giving the time that’s currently set for the quests to expire.
Oh, and iirc, at least for the few latest events, the timer has shown a time around midday, give or take an hour or two, EEST for the ending time, on the day it was marked to end. Don’t take that as an absolute truth, though, as it’s based on quick estimations and memory.