Thanks to scum-of-the-internet 4chan, we've all been hearing lately about a particular iOS bug that will brick modern iPhones, if you set the date back to 1/1/1970. Why does that happen? YouTuber Tom Scott explains. Scott gives a quick lesson in Unix time to explain why he thinks the bricking happens: for your iPhone, time is displayed as one single integer, representing the number of seconds since 1970. If you set the time back to 1 January 1970, that value becomes 0 — not a problem in and of itself, but if the phone tries to display a time before that, say a text you received a few hours ago, it will generate a negative number, which causes a crash.
This is only a theory — Apple hasn't confirmed what causes the bug, and probably never will. Even so, Scott's video is worth watching, both as a lesson in how computers intrepret time, and a cautionary tale for programmers everywhere.
If you've bricked your phone, the good news is it isn't gone forever: you need to either let the battery run entirely down (slow), pry the phone open and disconnect the battery (scary) or perform a Device Firmware Update (hard). Or take it to an Apple Store, and ask them nicely not to laugh at you.