How do I show a UTC time in local time using date-fns?

I know this has been asked before, but I have some kind of inability to process time zones and just can't get my head around it.

My React app is currently rendering a calendar, and I'm having difficulty with time zones. The dates of events are stored in UTC, and I retrieve a list of events and render them. I want to show the start time of each event in the user's local time, but I can't get it to work. I've also installed date-fns-tz, but it doesn't seem to be doing anything different.

Currently for each event I have a startDate in UTC, which looks like this: 2024-08-27T08:30:00.0000000.

If I display it using format(startDate, "HH:mm") I get: 08:30 (as expected).

However, in my time zone ("Europe/London") this event will actually start at 9.30 (my time), and that's what I want to display.

I can get the time zone from the browser using const tz = Intl.DateTimeFormat().resolvedOptions().timeZone;, but I'm not sure how to use that to convert and show the time.

I've tried using date-fns-tz's version of format: format(startDate, "HH:mm", { timeZone: "Europe/London" }) but still get 8:30.

I've also tried toZonedTime: toZonedTime(startDate, "Europe/London") which converts the startDate to Tue Aug 27 2024 08:30:00 GMT+0100 (British Summer Time) - I'm not sure if that's the correct time (British Summer Time is 1 hour ahead of UTC, so UTC+1). In any case, when I print it out, it still shows as 8:30.

Essentially, I have a startDate in UTC which is 8:30; however, when the event starts my watch will show 9:30, so 9:30 is what I want to display.

My trick is shift the Date object forward or backward in time to whichever time zone you need, which is equivalent to changing time zones.

You can shift a Date object into the browser's time zone using the following:

const convertToBrowserTimeZone = (utcDate) => {
    const browserTimeZoneOffsetMs = -60000 * (new Date()).getTimezoneOffset();
    return new Date(utcDate.getTime() + browserTimeZoneOffsetMs);
}

Then to format a UTC time in the browser's time zone:

format(convertToBrowserTimeZone(startDate), "HH:mm")