After you turn thirty, there are not many intermediate milestones before the big 4-0. While in your twenties you have 21 and even 25 (when you can rent a car), one's thirties seems to lack a certain number of calendrical landmarks. But, in fact, this isn't quite as checkpointless a decade as one might imagine. Not only is there an incredibly intriguing milestone that happens during one's fourth decade of life, but it provides a way of understanding the nature of a second, and how we accumulate them: one billion seconds.

Living one billion seconds occurs about two-thirds of the way between your 31st and 32nd birthdays. Specifically, one billion seconds is 31.69 years or a little more than 11,574 days. And my one billion seconds milestone occurs this coming Sunday, August 18th.

A billion seconds sounds like a long time: After all, a billion is a pretty big number. But as we can see, it's actually quite manageable, depending on how you look at it. And this is only one of the many quantities we are confronted with from our universe that can be viewed on the human scale, from the size of a neutron star (it could fit comfortable in the Boston metro area) to the frequency of supernovas in our galaxy (about once every hundred years). These are intersections of the cosmic with the human, and if you want to learn more about this topic, please check out "The Me-Sized Universe", an article I wrote several years ago on this topic.

But back to important matters: Want to calculate this one-billion-seconds milestone for yourself? If you want to do this on your own, make sure to take into account everything from leap years to leap seconds. Or, more conveniently, you can use Wolfram Alpha (which I assume handles all of these issues nicely) or this simple 1 billion seconds calculator to easily add 1 billion seconds to your birthday.

Top image: Kaja Kozłowska/Flickr/CC