The addition of an extra second to 2016 didn’t cause too much tech chaos, but did lead to some websites being difficult to reach for about 90 minutes.

A leap second is an additional second added at the end of a month to compensate for the fact that the Earth’s rotation is slowing down very gradually (and can also be altered by major seismic events) and thus slips out of synch with atomic clocks.

The International Earth Rotation and Reference Systems Service calculates when one is necessary and so far 27 leap seconds have been added (though only five in the past 18 years), all at the end of June or December. The second is officially added to the Cordinated Universal Time (UTC), used as a reference by most computer systems, by having an out-of-the-ordinary 23:59:60.

While the process is simple in concept, and comes with six months of warning, it can potentially cause problems with computers. While many machines work from remote time signals (which either literally broadcast the extra second or send an instruction to add a second), some simply can’t cope with the idea that a minute doesn’t have 60 seconds and need manual intervention. Popular Science notes that solutions include pausing the clocks for the final second or literally slowing down the clocks over the course or the day to spread the extra time out so it has no significant effect.

If the clocks don’t adjust, the problem isn’t so much that computers are running “slow” but rather that the disparity between different systems can cause confusion and computer logic problems. That’s what happened this year with Cloudflare, a company that reroutes DNS queries to help websites avoid DDOS attacks and other server load problems.

According to Cloudflare, the extra second caused one of the numbers in its software to turn negative, something the software wasn’t designed to cope with because the code writers worked on the sensible enough idea that time can’t run backwards. Part of Cloudflare’s DNS resolution process involves using time codes to generate a random number, a step which fell apart whenever a negative number was thrown into the mix.

Fortunately this only affected 0.2 percent of DNS queries and overall fewer than one percent of attempted visits to Cloudflare client sites threw up error messages.