Stop javascript Date function from changing timezone offset

Question

So I have iso date time that need to be converted from a string to date object. How do I keep date from converting it to local browser timezone.

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Answer 1

This is an old question that recently got linked in an answer to a newer question, but none of the existing answers seem to fully address the original question.

In many cases the easiest way to format and display a datetime string in the time zone that it is received, is not to convert the string to a Date object at all. Instead, just parse the datetime string and recombine the parts into the desired format. For example, if you can live without the day of the week in the output requested in the question, you could do something like the following to handle the requested input and output format (including the day of the week adds enough complexity that it might be worth using a library at that point). Of course, this approach requires modification based on the format of the datetime strings you are receiving on the client side.

const months = {1: 'Jan', 2: 'Feb', 3: 'Mar', 4: 'Apr', 5: 'May', 6: 'Jun', 7: 'Jul', 8: 'Aug', 9: 'Sep', 10: 'Oct', 11: 'Nov', 12: 'Dec'};

const format = (datetime) => {
  let [date, time] = datetime.split('T');
  let [y, m, d] = date.match(/\d+/g);
  let [t, tz] = time.split(/(?=[+-])/);
  return `${months[parseInt(m)]} ${d} ${y} ${t} GMT${tz}`;
}

let dt = format('2013-07-18T17:00:00-05:00');
console.log(dt);
// Jul 18 2013 17:00:00 GMT-05:00

There are JavaScript and other library methods that will allow you to convert your datetime string to a Date instance and then display it in a specific time zone if you are consistently receiving datetime strings from one or a few specific time zones that are easily identified on the client side. Following are a couple of examples.

toLocaleString enables you to display a date in a specific time zone but the options parameters required to set the time zone are not supported across all browsers (as of the date of this answer). For example (note that parsing of date strings with the Date constructor is still discouraged, but most modern browsers will handle an ISO 8601 format string like the one below):

const dt = new Date('2013-07-18T17:00:00-05:00');
const ny = dt.toLocaleString('en-US', { timeZone: 'America/New_York' });

console.log(`Local: ${dt}`);
console.log(`New York: ${ny}`);

You could also use Moment.js with Moment Timezone to display a date in a specific time zone. For example:

const ny = moment('2013-07-18T17:00:00-05:00').tz('America/New_York').format()

console.log(`New York: ${ny}`);

It is important to note the following when working with datetime strings and time zones in JavaScript. The JavaScript Date constructor DOES NOT change the time zone offset. When you create a date from a string using ISO 8601 format as in the question example, Date does the following:

Creates a JavaScript Date instance that represents a single moment in time. Date objects use a Unix Time Stamp, an integer value that is the number of milliseconds since 1 January 1970 UTC.

In other words, JavaScript Date instances do not store (or change) time zone offsets. The reason you often see output from a JavaScript Date in your local time zone is that toString is the default method called when you log a Date instance to the console (or use various other approaches to output your date). In most browser implementations, this method relies on the browser's local time zone to convert the number of milliseconds since 1 January 1970 UTC into a string representation of the date.