Change to BST has invalidated my SSL cert

Hello, I have had my nextcloudpi server running flawlessly for five months, syncing my iPhone etc.
Today I tried logging into it using firefox and got the SSL error page which included the following:
“The web site is either misconfigured or your computer clock is set to the wrong time.”
Ssh-ing into the server I noticed it was at the correct time but using UTC rather than BST which would have been the same as GMT, probably why its worked until now.
All my other locale settings were correct so I set my tzdata to correct BST and rebooted. Still no joy, any suggestions?

NextCloudPi v1.10.6
Nextcloud version 15.0.5
Operating system and version: Raspbian latest, upto date

Normally, you can run your server in the timezone you prefer as long as you have set the right timezone (and the time/date is right). Such errors normally appear, when there is a difference between the client and the server time (they respect different timezones, so no problem if the client runs local time and the server utc or other time zone). Please note that the client time zone can be wrong as well (after empty battery etc.). Don’t forget to check the date as well.

Thanks for replying, Strange that after updating the time zone did not fix it (the server time and date were correct) had the same result from any way I accessed it including iphone app so I dont think it was that the client time date was wrong. Fortunately it is so easy to redeploy nextcloudpi and restore the backed up data that a rebuild fixed it without too much hassle!