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
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 date command

date 1201160419    # Sets date to 4:04PM Dec 1, 2019
Quit terminal and restart install procedure