I have the official emoncms docker image running and all graphs are in UTC despite the correct timezone being set in php.ini and in the account settings. The timezone in the container is UTC. I am not sure what else I can check.
Edit: the Administration page shows correct Timezone under Server->Date.
It is used when data requests are made for feed data that aligns to day, week, month. The returned data will align to midnight of the timezone set in the user account profile. It is also used by the power_to_kwhd input processor to find the day reset point.