Why One Second Brought Down Cloudflare DNS: The Hidden Complexity of Time
At midnight UTC on January 1, 2017, deep inside Cloudflare’s custom RRDNS software, a number went negative when it should have always been at least zero. This single value caused DNS resolutions to fail across Cloudflare’s global network. The culprit? A leap second—one extra tick of the clock that most people never noticed. The bug revealed a fundamental truth that every programmer eventually learns the hard way: time is not what you think it is. It doesn’t flow uniformly forward. It jumps, skips, and occasionally rewinds. And if your code assumes otherwise, it will break in ways that are nearly impossible to predict. ...