A bug in iOS 11.1.2 is causing iPhones to crash repeatedly once the clock hits 12.15am on December 2nd. As noted by iMore and Twitter engineer Yoshimasa Niwa, the bug appears to be related to third-party apps that use recurring local notifications for things like reminders.
As such, you have two immediate options to stop the crashes if you’re finding yourself affected: turn off notifications, or set your date back to some point in time before December 2nd, 2017. You can also use the public beta of iOS 11.2, which apparently fixes the issue.
These are all pretty drastic workarounds, but if you do find yourself affected as December 2nd rolls around in your timezone — I didn’t, myself — they should work as quick fixes until Apple releases the final version of iOS 11.2.