Proton mail uses the browser time zone for all events and there is not a way to set it manually. I use Librewolf to minimize browser fingerprinting and one of it's features (which is not easy to turn off separately) is that is displays the time zone to websites as Zulu / GMT, instead of local time. This means that calendar events come in with a time shift displayed in email. They are correct in the calendar since Calendar allows you to set the time in the top bar.