All content in Magnolia is stored in the default time zone. By changing the time zone in user profile you control how the default time zone is rendered in the UI.
The server uses the default time zone (the server time) to store the dates you enter in date/time fields. Consider the following example.
If both server and user time zones are set to CET (default, UTC+1) and in the period of the year which observes the daylight saving time you enter 2019-07-12 04:00 in a custom field called myDate, the value will be saved to JCR as follows:'myDate': 2019-07-12T04:00:00.000+02:00
However, if server time zone is CET but user time zone is PST (UTC-8), and you enter the same value (2019-07-12 04:00) in the field, Magnolia will not store the value as'myDate': 2019-07-12T04:00:00.000-07:00
but rather as a transcoded value seen from the server's perspective, keeping the server's UTC offset as the baseline:'myDate': 2019-07-12T13:00:00.000+02:00