This plugin allows you to set the php and the mysql timezones. phplist lets you set the timezone using a config file parameter, SYSTEM_TIMEZONE, but that has to be a value that is valid for both php and for mysql, such as 'Australia/Sydney'. This requires mysql to support time zone information tables, which often is not the case.
This plugin sets the mysql timezone using an offset calculated from the php timezone.
See the README file on the GitHub page https://github.com/bramley/phplist-plugin-timezone for how to install and configure the plugin.