How to manipulate date with a different timezone then convert it to UTC?

Question

So, I live in Philippines (GMT+8) my current time is June 1, 2020, 3:20 PM

The current UTC time is June 1 2020, 7:20 AM.

I want it so I can manipulate the date using Pacific/Honolulu (GMT-10) and the current time there is 31 May 2020, 9:20 PM like:

const date = new Date();
date.setDate(l.getDate() + 1)
console.log(date)
// ...further manipulation of date

date.getTime() //unix timestamp

this will show the local timezone which is GMT+8.

So I want to choose a timezone, manipulate the date, then get the UTC timestamp of that manipulated date.

I tried various method like converting it first to UTC, but no luck - still can't seem to find any workaround to this.

I'm using timezone-support and date-fns (if this helps) in my project (sorry I can't use moment js) as the project is already quite big and is using date-fns for a long time.

Answer 1

So I ended up doing this since was using timezone-support:

first I imported and initialized the data:

import {
  populateTimeZones,
  listTimeZones,
  getZonedTime,
  findTimeZone,
  getUnixTime,
  setTimeZone,
  convertTimeToDate,
} from 'timezone-support/dist/lookup-convert';

import timezoneData from 'timezone-support/dist/data-2012-2022';

// get the timezone support ready
populateTimeZones(timezoneData);

Then I created this helper function:

const dateConvertToTimezone = (fromTimeZone, toTimeZone, date = new Date()) => {
  const tz = findTimeZone(fromTimeZone);

  const time = {
    hours: date.getHours(),
    minutes: date.getMinutes(),
    day: date.getDate(),
    month: date.getMonth() + 1,
    year: date.getFullYear(),
  };

  // set the timezone to get the time object for the selected timezone
  const selectedTime = setTimeZone(time, tz);

  // convert the timezone to local one
  const convertedToLocal = convertTimeToDate(selectedTime);

  const utc = findTimeZone(toTimeZone);
  const convertedTime = getZonedTime(convertedToLocal, utc);

  return convertedTime;
};

Then I can do something like:

const selectedTime = dateConvertToTimezone(
 'Etc/UTC',
 'Pacific/Honolulu',
 new Date('May 31 2020, 21:20')
);

This will return the UTC for Honolulu time and not my local time.