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.

_=>new Date/12e5%72<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.

05AB1E, 32 26 11 9 8 bytes

žažb«21‹

Explanation:

ža          Is the current hour &
  žb        current minute
     «      concatenated
        ‹   less than
      21    twenty one?

Try it online!

Python 2, 41 39 bytes

Saved 2 bytes thanks to Erik the Outgolfer

import time
print time.time()/1200%72<1

Try it online!

Uses the same algorithm as my JS and Charcoal answers.