I'm getting this today, I've never had this before.
[minepeon@minepeon minepeon]$ git pull
fatal: unable to access 'https://github.com/MineForeman/MinePeon.git/': SSL certificate problem: certificate is not yet valid
Think you have an issue with the date on the Pi
date
sudo ntpd -s
Since I use wifi only, I'll put in my 2 cents, I had to make sure openntp was enabled so that time would update on reboot as well:
sudo systemctl enable openntpd.service