A megasecond, one million seconds, is 11 days and 14 hours. (13 hours, 46 minutes and 40 seconds, assuming no leap seconds.) About a week and a half. Ten megaseconds is 116 days. (115, plus 17:46:40.) A bit less than four months, a third of a year. A hundred megaseconds is 1,157 days. (Plus 9:46:40^{1}.) Roughly three years and two months—it could be 61 or 62 days depending on leap years.
I think it might be kind of nice if we celebrated a hundred megaseconds additonallyto and/or insteadof birthdays.
One thing I like about it is that humanity won’t stay on Earth forever. If we go interplanetary, I think we’re more likely to keep measuring time in seconds than in Earth years. I like this as a way of looking to the future.
Another thing is, it doesn’t happen at the same time every (Earth) year. Some birthdays fall on or near holidays, or during seasons we don’t much like, and that can make them less convenient to celebrate. If that happens to you one hundred megaseconds, it probably won’t happen the next hundred megaseconds. (I think a five month offset would be better than two for this purpose, you’d never be in the same season twice in a row, but two is better than zero.)
I also just think it’s kind of a sweet spot time interval? I’m old enough now that years don’t feel very long, decades^{2} do, and a hundred megaseconds is nicely in between.
(If “hundred megaseconds” is unwieldy, a quick look at wikipedia suggests “yisec” from Chinese or “okusec” from Japanese. Or maybe “myrimyrisec” from Ancient Greek. I think my favorite would be “hectomegasec” from Ancient Greek via SI. I’m not going to try to use any of these myself though.)
Ten hundred megaseconds, a billion seconds, is 11,574 days. About 31 years eight months. (The remainder is 251253 days, depending on leap years^{3}. Plus 1:46:40.) That’s how old I turned on August 21st this year.
I didn’t do anything to celebrate, partly because even though I’d put it in my calendar a long time before, I didn’t remember until a couple days after, and partly because I’m not very good at celebrating things. (I don’t normally celebrate my birthdays, either.) I did do some work on Haskenthetical for the first time in months, which feels like a good start to the next billion. Maybe I’ll celebrate turning eleven, which will be on October 21st, 2024.
If you’re interested, I made a widget you can use to calculate your age, or other intervals, in hundred megaseconds. It won’t work on LW, but you can use it at the original site.
Thanks to Miranda for comments.

Why are the minutes and seconds always the same? Because 46 minutes and 40 seconds is 2800 seconds, and the remainder when dividing (10 × 2800) by (60 × 60) is also 2800. So multiply the previous duration by 10 and you get the same number of minutes and seconds left over. For similar reasons, the number of hours after hitting 10 megaseconds will keep looping: 17, 9, 1, 17, 9, 1. ↩

Incidentally, a decade would be π hundred megaseconds, to within 0.4%. ↩

It would be 259 days if there were no leap years. You’d normally experience February 29th eight times in that period, but it might only be seven if you start shortly after a February 29th. And it might only be six, if the period crosses a 100butnot400year boundary. ↩
I always thought a standard day would be a better base time than seconds. Human circadian rhythms can only be stretched so far.
For now—growth mindset!
But apart from that, a Martian day is only 40 minutes longer than an Earth day. So I expect it would still be fine for current human circadian rhythms, and measuring time in Earth days would be inconvenient for Martians.
It’s been done. https://en.wikipedia.org/wiki/Decimal_time
That aside, what’s so special about base 10?
Wouldn’t it be more logical to celebrate passage of, say, 2^24 seconds (194 days)? Once we’re no longer constrained to human bodies, we can have as many fingers on our hands as we want, so we may as well have 8 or 16.
The objectively correct base is six.
The human lifespan currently is limited to a long: 2^32 = 136 years (google can answer that).
Sorry, but outside of surface elements (being about time, base ten, the word “second”) I’m not sure what about my proposal you think has been done in decimal time.
There’s nothing particularly special about base ten¹, but I’m also not sure there’s any particular reason to move away from it, much like the second. Base twelve would probably be more convenient for mental arithmetic, but I don’t much expect a shift to occur. If 2^24 seconds would be more logical, I confess I don’t see why—what do you mean by “logical”?
And, like… this is partly about looking to the future, but I can’t just ignore the present. Seconds mean something to people today. A billion seconds means something to people today. 2^24 or 2^30 seconds might mean something to some people, but not as many.
Base 10 is special because every base is base 10. :p