If you're on terra firma, it's pretty obvious when you need to grab your make-out partner. But how do you know when to celebrate "when you're hurtling through time zones at 28,164km per hour?"
The obvious answer is something like: "you don't pick a single moment, you celebrate once over each of the 30 different time zones in an all-day, ISS, kegger of space beer blow out.
The actual answer is a little less exciting. The ISS astronauts set their clocks to Coordinated Universal Time, which is the same thing as Greenwich Mean Time, just with a cooler name.
But they still got some time zone-hopping fun: the ISS crew celebrated New Years with mission controllers in Moscow and Houston when the clock struck midnight in those cities. [Slate's Explainer]