UNTRUSTED_CERT_TITLE trying to install Sierra
I'm trying to reinstall macOS Sierra from the recovery partition, and I'm getting an error when it is checking for eligibility 
The computer that I'm using is most definitely a Mac, and I can't use it until I install Sierra
Solution 1:
Just to give an actual set of steps to expand on the answer from Sam
- Go to Utilities -> Terminal
-
If connected to Wifi, use ntpdate
ntpdate time.apple.com
-
If not on wifi, use
datecommanddate 1201160419 # Sets date to 4:04PM Dec 1, 2019
Quit terminal and restart install procedure