Timezones can be tough, but they are extensively talked about in the forums. Here is a post from just a few days ago where @cmarchan linked the arguably best resource for understanding bubble timezones.
Give it a read, as it really helps.
But I also mentioned a Bubble feature in the Settings that overrides all timezones when saving date/times to the database . You can use this feature at the workflow-level so that the dates which are specifically being used for the calendar don’t get affected by timezones (so they just represent dates rather than date/times)