Certificate is not valid yet

What's going on

The certificate's start date is in the future, so clients refuse it. Nine times out of ten the certificate is fine and a system CLOCK is wrong — either on the machine checking it or the machine that issued it.

How to fix it

Check the clock, time zone and date on the machine reporting the error first (Settings → Time & language → sync now). If the clock is right, compare the Not Before date against reality — a certificate issued moments ago can be 'not yet valid' for a few minutes on machines with slight clock drift.

New to certificates? Start with the guide: How to read an SSL certificate.