I'm not certain, or the web-client has a wrong time, or the server has a wrong time, or the "encrytion-key" on the server is outdated , or is self-signed , or could not be hierarchically validated.
If you really trust the server you can add an "exception" in the web-browser to forcefully accept the encryption.
For a "normal" site you visit this is mostly not a problem to accept the exception.
But don't do it when you have to enter "personal or financial data".
PS : Do you have an ntp client running on FreeBSD with ntpdate ? [ This to be certain your system time is correctly synchronized].