Miles are based on date flown, not credited. You’ll earn your status.

But, 20k miles in a single day? Something doesn’t add up... How are you doing this?

I don’t know how you will manage to get 20K AQM on one leg but if all individual segments of your leg to Taiwan depart on December 31st or before, you’ll get it. If one of the flight departs on January 1st on the land of departure, this particular flight will count toward the following year.

What’s you itinerary?