It should normally be set correct automatically if the options.conf is set correctly.
To be sure just check. In options.conf the first php option is used for the system. So for example if php1 is php 7.4 then the according php.ini would be in the /usr/local/php74/lib directory.
If it's correct there too, check in Roundcube if there it's still set to automatically and somebody did not change it.
This can be done by loggin in to Roundcube, click settings, then User Interface. It should be at "automatic" there.
If those are all the case check your /etc/localtime if that is set or symlinked to the correct timezone.
What OS are you using?