timezone offsets incorrect when environment timezone changes

[ben-cook]

Dates created after a change in the surrounding TZ environment variable have incorrect results from getTimezoneOffset(). This could be related to the cache used in tzOffset - I've noticed that the offsetFormatCache used internally gets an entry for when the timezone is undefined

I've created a Code Sandbox to demonstrate this issue, and if that doesn't work I've pasted the code below:

import { TZDateMini } from "@date-fns/tz";

process.env.TZ = "America/Los_Angeles";
let implicitLosAngelesDate = new TZDateMini(2024, 0);
// Returns 480, which is correct
console.log(implicitLosAngelesDate.getTimezoneOffset());

const explicitMelbourneDate = new TZDateMini(2024, 0, "Australia/Melbourne");
// Returns -660, which is correct
console.log(explicitMelbourneDate.getTimezoneOffset());

process.env.TZ = "Australia/Melbourne";
const implicitMelbourneDate = new TZDateMini(2024, 0);
// Should be -600, but returns 480 instead
console.log(implicitMelbourneDate.getTimezoneOffset());

The use case I had when I ran into this bug was unit testing a complex application in a variety of browser timezones, simulated in a node script. Running consecutive tests with changes to the TZ environment variable wouldn't pick up on the changes to the environment variable, which would cause non-deterministic behaviour between running tests all at once vs running the tests one-by-one.