JavaScript utility to shift a date to UTC

shift-date-to-utc.js

/*
Note: if you only want to format a date as a string and not do any date
manipulation, you can use provided formatting functions to specify a time zone:

    const date = new Date(dateString);
    const formattedDate = date.toLocaleDateString('en-US', { timeZone: 'UTC' });

All JavaScript Date objects are in the browser's local time zone. There's no way
around that. This means that if you receive a date string from the backend and
create a Date object with `new Date(unixTimestampOrISOString)` that Date object
will potentially be on a different calendar date than it is in UTC.

Here's a demonstration of this problem with a computer in Pacific Daylight Time.
Passing individual year, month, and day arguments to the Date constructor works
fine, but passing a date string results in a Date object on a different calendar
date:

    new Date(2022, 8, 30)
      Fri Sep 30 2022 00:00:00 GMT-0700 (Pacific Daylight Time)
    new Date('2022-08-30')
      Mon Aug 29 2022 17:00:00 GMT-0700 (Pacific Daylight Time)

This function will essentially shift the Date object to the correct UTC year,
month, and day. But note, the Date object that's returned will still *think*
that it's in the local time zone, so e.g. if you apply this function multiple
times it will keep shifting.
*/

function shiftDateToUTC(date) {
  const utcDate = new Date(
    date.getUTCFullYear(),
    date.getUTCMonth(),
    date.getUTCDate(),
    date.getUTCHours(),
    date.getUTCMinutes(),
    date.getUTCSeconds()
  );
  return utcDate;
}