Basically, a Beddian birthday is when you are the same age as last two digits of the year you were born. But a little trip down history lane reveals there’s a quite bit more to it than just that. We recommend busting out an ice cream cake for this one. It’s quite a ride.

On November 5, 2007, The New Yorker published an article written by Lizzie Widdicombe entitled, “The Firefighter’s Theorem.” This article recounts a story about Bobby Beddia, a New York firefighter, and his now-famous birthday observation. On a sunny August Saturday, Bobby chatted with Rhonda Shearer, a visitor to his fire station, about his great luck in getting to live during his birth year. When asked to elaborate, he explained that he was born in September 1953, so his age in 2006/2007 matched the final two digits of the year in which he was born: 53. This phenomenon happens to everyone, but only once in a lifetime, and Bobby Beddia was thrilled to be experiencing his. Tragically, later that day, Bobby was killed in the line of duty.

As a tribute to her friend, Rhonda decided to investigate this birthday observation, and the result was some pretty fascinating math—if you’re into that sort of thing. She reached out to her physicist and mathematician friends for more insight, and the “birth year” eventually came to be known as a Beddian year. Richard Brandt, a former NYU physicist, observed that Beddian birthdays will only ever happen in even-numbered years. That’s because, mathematically, the event is essentially a doubling. Think of it this way: if you were born in 1960, your Beddian year is 2020, the year you turn 60. 1960 + 60 = 2020. If you were born in 1953, your Beddian year was 2006: 1953 + 53 = 2006. Do you see the doubling effect? One of the most basic math concepts is that the result of doubling any number is always an even number; hence Beddian birthdays are strictly an even year occurrence.

Mathematician Barry Cipra took the math one step further. He posited that, at any given time, everyone in the world is either pre- or post-Beddian in their age; in other words, everyone has either already had their Beddian birthday, or they haven’t had it yet. With that in mind, he wanted to know which group was larger, and his answer to this question is what is now called the Beddia Theorem.

“In any odd-numbered year, there are exactly 50 pre-Beddian ages. In any even-numbered year, there are exactly 49 pre-Beddian ages. Moreover, with three exceptions, these ages consist of two separate spans. The exceptions are 1998 (or any year ending in ’98), for which the pre-Beddian ages comprise the single span 0-48, 1999 (or any year ending in ’99), for which they comprise the single span 0-49, and 2000 (or any year ending in ’00), for which they comprise the single span 1-49.”

If you managed to get this far, we assume your Ice Cream Cake didn’t make it through the journey with you. But what’s the takeaway from all this? Beddian years only come around once in a lifetime. Everyone gets one; noticing that yours has arrived is a very big deal. And really, you can celebrate a Beddian birthday all year long!