[javascript] How do you convert a JavaScript date to UTC?

My solution keeps the date the same no matter what timezone is set on the client-side. Maybe someone will find it useful.

My use case:

I'm creating a todo app, where you set date of your task. This date should remain constant no matter what timezone you're in.

Example. You want to call your friend at 8 am on June 25th.

You create this task 5 days before (June 20th) while you're in China.

Then, on the same day, you fly to New York for a few days.

Then on June 25th, while you're still in New York, you wake up at 7:30 am (which means you should receive task notification in 30 mins (even tho it's 1:30 pm already in China where you were when creating the task)

So the task is ignoring the timezone. It means 'I want to do it at 8 am in whatever timezone I'll be in'.

What I do is let's say 'I assume you're always in London Timezone - UTC'.

What it means is - when the user picks some date in her/his Timezone - I convert this date to the same date in UTC. ie. You pick 8 am in China, but I convert it to 8 am in UTC.

Then - next time you open the app - I read the date saved in UTC and convert it to the same date in your current timezone - eg. I convert 8 am in UTC to 8 am in the New York timezone.

This solution means that the date can mean something else depending on where you are when setting it and where you're reading it, but it remains constant in a way that it 'feels' like you're always in the same timezone.

Let's write some code:

First - we have 2 main functions for converting from/to UTC ignoring timezone:

export function convertLocalDateToUTCIgnoringTimezone(date: Date) {
  const timestamp = Date.UTC(
    date.getFullYear(),
    date.getMonth(),
    date.getDate(),
    date.getHours(),
    date.getMinutes(),
    date.getSeconds(),
    date.getMilliseconds(),
  );

  return new Date(timestamp);
}

export function convertUTCToLocalDateIgnoringTimezone(utcDate: Date) {
  return new Date(
    utcDate.getUTCFullYear(),
    utcDate.getUTCMonth(),
    utcDate.getUTCDate(),
    utcDate.getUTCHours(),
    utcDate.getUTCMinutes(),
    utcDate.getUTCSeconds(),
    utcDate.getUTCMilliseconds(),
  );
}

Then, I save/read this date like:

function saveTaskDate(localDate: Date) {
  // I convert your local calendar date so it looks like you've picked it being in UTC somewhere around London
  const utcDate = convertLocalDateToUTCIgnoringTimezone(localDate);
  api.saveTaskDate(utcDate);
}

function readTaskDate(taskUtcDate: Date) {
  // I convert this UTC date to 'look in your local timezone' as if you were now in UTC somewhere around london
  const localDateWithSameDayAsUTC = convertUTCToLocalDateIgnoringTimezone(taskUtcDate);

  // this date will have the same calendar day as the one you've picked previously
  // no matter where you were saving it and where you are now
}