On my Nexus 6P, Android 7.1.2, I once entered a Do Not Disturb setting which started at 0450 local time. I deleted that, but since the 7.1.2 upgrade, it has returned. It even occurs in Safe mode. How do I find it, and kill it, dead, with a stake through its ID?
DND is not a cron job... It's "embedded" in the OS databases. Unless someone knows exactly what file and where, assuming it's ASCII editable file, a factory reset might be your only option.
If you rooted great if not that's okay.
Download terminal emulator and type
top you will see a list of of processes find the one you are looking for and follow the on screen instructions on how to kill it.