It's all down to the rotation of the earth. As we take stock of today being the final one of 2012, it's interesting to reflect on which part of the world will see the dawn of 2013 when.
Christmas Island and Samoa get there first, when it will still only be 5am on New Year's Eve in New York and 10am in London. The last? The Midway Islands - their New Year's Day doesn't begin until a full five hours after New York's bells have chimed (and ten hours after Big Ben has marked midnight in London).
It's complicated, but we'll all get there in the end.
Some will be at parties tonight, perhaps celebrating with friends. Others will be at home alone: perhaps wishing they weren't or, maybe, secretly relieved that they can do exactly as they please, in the knowledge that there may be the opportunity for a lie-in tomorrow morning.
However, the idea of seven billion people moving from one year to another is a good reminder of the fact that you and I are unquestionably part of something bigger, and it can be good to remind ourselves of this from time to time.
I know, all too well, that when you go through a rough patch it can feel as if you're on your own. It may seem a lonely life.
On better days, you'll maybe recognise that this is rarely the real case for anyone. But this won't stop you feeling isolated.
So although I really hope it isn't the case, if you do happen to be heading towards January 1st with a heavy heart, please remember that among the seven billion are certain to be some who care for you, and for whom you care. There are some who can help you, and others who may need your help. And there are those who love you, and you love back.
It's good to be part of something bigger, and they don't get much larger than 7,000,000,000.