When will SE be down?
JavaScript (ES6), 26 24 23 22 21 bytes

Saved 3 bytes thanks to Shaggy and 1 byte thanks to Luke.

_=&gt;new Date/12e5%72&lt;1


Checks if time passed in current day is less than 1200000ms (1200s or 20min). Assumes downtime to be 20 minutes not 21, which appears to be the case in the linked post. 00:20UTC is the exclusive upper bound.