Format a date to ISO string with timezone using JavaScript

The ISO 8601 standard defines a format for dates and times. It is widely adopted across the world and has applications in all fields, from business to science.

Dates in the format are represented either as:

• YYYY-MM-DDTHH:mm:ss.sssZ (UTC time)
• YYYY-MM-DDTHH:mm:ss.sss±hh:mm (local time with timezone offset)

Let's see how we can convert a date to either format using JavaScript.

Format date to ISO string (UTC time)

JavaScript's built-in Date.prototype.toISOString() method converts a date to ISO string in UTC time.

const toISOString = date => date.toISOString();

toISOString(
  new Date('2024-01-06T19:20:34+02:00')
); // '2024-01-06T17:20:34.000Z'

Format date to ISO string with timezone offset

In order to include the timezone offset, you have to build the string yourself.

For this, you'll need to know the timezone offset of the date, which can be retrieved using Date.prototype.getTimezoneOffset(). The returned values is in minutes, so you'll have to convert it to hours and minutes. It also returns a negative value for positive offsets, so you'll have to reverse the sign.

On top of that, you'll have to manually pad the values to 2 digits, using String.prototype.padStart(). Using that, in combination with the timezone offset and the built-in methods in Date, you can build the ISO 8601 string with timezone offset.

// Pad a number to 2 digits
const pad = n => `${Math.floor(Math.abs(n))}`.padStart(2, '0');
// Get timezone offset in ISO format (+hh:mm or -hh:mm)
const getTimezoneOffset = date => {
  const tzOffset = -date.getTimezoneOffset();
  const diff = tzOffset >= 0 ? '+' : '-';
  return diff + pad(tzOffset / 60) + ':' + pad(tzOffset % 60);
};

const toISOStringWithTimezone = date => {
  return date.getFullYear() +
    '-' + pad(date.getMonth() + 1) +
    '-' + pad(date.getDate()) +
    'T' + pad(date.getHours()) +
    ':' + pad(date.getMinutes()) +
    ':' + pad(date.getSeconds()) +
    getTimezoneOffset(date);
};

toISOStringWithTimezone(
  new Date('2024-01-06T19:20:34+02:00')
); // '2024-01-06T19:20:34+02:00'

Check if a string is an ISO formatted date

As often as you might need to convert a date to ISO format, you might also need to check if a string is a valid ISO formatted date. Knowing how to convert a date to ISO format, you can simply compare the result with the original string.

Add a check for invalid dates with Number.isNaN() and a date conversion, using the Date constructor, and you're ready to.

const isISOString = val => {
  const d = new Date(val);
  return !Number.isNaN(d.valueOf()) && d.toISOString() === val;
};

const isISOStringWithTimezone = val => {
  const d = new Date(val);
  return !Number.isNaN(d.valueOf()) && toISOStringWithTimezone(d) === val;
};

isISOString('2020-10-12T10:10:10.000Z'); // true
isISOString('2020-10-12'); // false
isISOStringWithTimezone('2020-10-12T10:10:10+02:00'); // true
isISOStringWithTimezone('2020-10-12T10:10:10.000Z'); // false