Bitcoin’s halving event is set for Friday or Saturday—why it’s hard to say exactly when

It’s not exactly a solar eclipse but, in terms of the crypto world, it’s rare and miraculous in its own right. I’m talking about the impending Bitcoin halving, which is designed to take place every four years or so, and this time around will result in the supply of each new block of the blockchain delivering 3.125 new Bitcoins instead of 6.25. Plenty has been written about what this will do for the price of Bitcoin (Niamh Rowe has a good summary here), so I wanted to write instead about why no one can say for sure when this big event will happen.

Earlier news reports speculated that the Bitcoin halving would take place on Saturday 4/20, which would offer the opportunity for plenty of “all-time-high” jokes. But more recently, reports have hedged by saying the event will occur either Friday or Saturday. The precise reason for the uncertainty is well known to hard-core Bitcoin folks but probably not to most of those who follow crypto more casually.

The answer lies in the fact that new blocks are not added to Bitcoin’s blockchain at precise intervals. While it’s common to say new blocks are added every 10 minutes, that’s just a rough estimate, since adding blocks requires solving a trial-and-error math problem and the time it takes to do that is unpredictable. To see for yourself, check out a Bitcoin explorer site like this one.

The site currently shows a countdown clock to the halving. As I write this, the clock shows the most recent block to be added as 839,940—which is 60 blocks away from the halving, which will take place at block 840,000. You can also see the time when new blocks are added and the interval times. As the recent chronology shows, some blocks took as long as 25 minutes to arrive while at other times the next block was added in less than two minutes. (This variance is normal and, as people very familiar with Bitcoin know, the software’s design includes a function that makes the random math problem harder or easier in order to ensure the pace of new blocks stays close to a 10-minute average.)

The upshot is that the Bitcoin halving will certainly take place later today—or tomorrow. All of this is probably not the sort of thing you should try to share at a dinner party, but I bring it up as a reminder of the ingenuity and elegance of Satoshi Nakamoto’s creation. Bitcoin is now 15 years old, and as its fourth halving fast approaches, it continues to run like a Swiss watch. Happy halving to those who celebrate—and happy 4/20 to those who celebrate other things. Have a great weekend.

Jeff John Roberts
jeff.roberts@fortune.com
@jeffjohnroberts

This story was originally featured on Fortune.com