If you're on terra firma, it's pretty obvious when you need to grab your make-out partner. But how, asks Slate's Explainer, do you know when to celebrate "when you're hurtling through time zones at 17,500 miles 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, space beer kegger.
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. That means they were debating if it's "two thousand and ten" or "twenty ten" at the same time Londoners were doing so back here on Earth.
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]