We celebrated my wife’s 101100th birthday over the weekend. No, she’s not into her 101st millennium. She’s simply been denoting them in binary since her 101000th birthday or so. But it got me to thinking about birthdays on colony worlds and how those will be calculated.

Take Mars as a simple example. The Martian year is 687 Earth days, and with a day a little longer than 24 hours, it’s only 668 Martian days. So, if you were born on the first day of the new Martian year, when are you one year old? When do you get to celebrate your birthday?

It really comes down to which calendar you keep, but even then, you may not keep the same calendar for everything. New Years really only comes to Mars once per Martian year, but maybe Christmas comes twice. Martian months may not make much sense with Phobos and Deimos, but they were always a little arbitrary on Earth as well. Martian Independence Day is certainly celebrated every Carter 4th, but when do you have Thanksgiving?

There’s certainly a temptation to go full in one direction or another, i.e. stick with the Earth calendar or go native with the local calendar, but if there’s enough interaction between Earth and the colony, there’s some value in going halfway. Any joint schedule between the two worlds (or if we go interstellar, dozens of worlds) should be on a shared calendar, and since the Earthers had theirs first, this standard calendar should be based on the Earth year.

Another reason for using a standard reference calendar is a bit closer to this question of your birthday. When do your six-year molars come in on Mars? What about on Ganyemede? When will you go through puberty? Who is older, you or your cousin from Europa? There are plenty of biological reasons to keep track of our age in a standardized fashion. Even if we get our tetanus booster shot every five years on Mars, we do it because it was every ten years on Earth.

But still, when do you have that birthday party?

Are you on Mars all the time, or are you rocketing off to Jupiter or Saturn on a regular basis? It’s tempting to say that if you stay in one place all the time, you may as well celebrate it according to the local calendar. Then again, 668 days is a long time to wait for a celebration on Mars. Even worse is the poor kid growing up on Titan, orbiting around Saturn. He’ll be married with kids of his own before his first birthday. Hmm, maybe the local calendars aren’t such a good idea after all. But in some other solar system, on an Earth-like world with seasons of its own, following some arbitrary “standard calendar” for your birthday sounds silly.

I suspect that the real answer is that you’ll celebrate your birthday whenever it makes sense. If your local calendar’s year is only ninety-four days – and short ones at that – then the local custom might evolve to have quadrennial celebrations for your birthday. If you’re dragging yourself around that local star every fourteen hundred days, the local custom might be to celebrate your spring birthday followed by your summer birthday and so on, four celebrations each year. Only those folks skimming through the universe on ships will celebrate birthdays on the standard year.

Or maybe the whole concept of birthdays will fade away as one of those old Earth customs that seem silly to the post-human immortals who live amongst the stars.

What do you think?