ISO 8601 Format Dates with Timezone Offset in JavaScript

In JavaScript, constructing dates in ISO 8601 format with timezone offset can be challenging due to potential negative timezone offsets. This article addresses this issue and provides a solution.

Understanding the Format

The ISO 8601 format specifies dates as follows: YYYY-MM-DDThh:mm:ss±hh:mm. For example, 2002-10-10T12:00:00-05:00 represents noon on October 10, 2002, in Central Daylight Savings Time (five hours behind UTC).

Finding Local Time and UTC Offset

To construct the ISO 8601 string, we must first obtain the local time using new Date() and calculate the UTC offset using getTimezoneOffset(). The offset is obtained in minutes, so we divide it by 60 to get the number of hours.

Handling Negative Timezone Offsets

The getTimezoneOffset() function can return negative values. In such cases, we must format the offset differently. For example, an offset of -120 minutes should be displayed as 02:00 (two hours ahead of UTC).

Helper Function for Formatting

To simplify the process, a helper function, toIsoString, can be utilized to format dates in ISO 8601 format with timezone offsets:

function toIsoString(date) {
  var tzo = -date.getTimezoneOffset(), // Make the offset positive
      dif = tzo >= 0 ? ' ' : '-', // Determine the sign
      pad = function(num) { // Ensure two-digit representation
          return (num 
This function takes a date as an argument and formats it according to the ISO 8601 specification, including the timezone offset.
Example Usage
The following code demonstrates how to use the toIsoString function:
var dt = new Date();
console.log(toIsoString(dt)); // Outputs the date in ISO 8601 format with timezone offset
Using this approach, you can easily format dates in JavaScript according to the ISO 8601 standard, ensuring that they adhere to the proper format.