How to ISO 8601 format a Date with Timezone Offset in JavaScript?

Goal: Find the local time and UTC time offset then construct the URL in following format.

Example URL: /Actions/Sleep?duration=2002-10-10T12:00:00−05:00

The format is based on the W3C recommendation. The documentation says:

For example, 2002-10-10T12:00:00−05:00 (noon on 10 October 2002, Central Daylight Savings Time as well as Eastern Standard Time in the U.S.) is equal to 2002-10-10T17:00:00Z, five hours later than 2002-10-10T12:00:00Z.

So based on my understanding, I need to find my local time by new Date() then use getTimezoneOffset() function to compute the difference then attach it to the end of string.

1. Get local time with format

   var local = new Date().format("yyyy-MM-ddThh:mm:ss"); // 2013-07-02T09:00:00
   

2. Get UTC time offset by hour

   var offset = local.getTimezoneOffset() / 60; // 7
   

3. Construct URL (time part only)

   var duration = local + "-" + offset + ":00"; // 2013-07-02T09:00:00-7:00
   

The above output means my local time is 2013/07/02 9am and difference from UTC is 7 hours (UTC is 7 hours ahead of local time)

So far it seems to work but what if getTimezoneOffset() returns negative value like -120?

I'm wondering how the format should look like in such case because I cannot figure out from W3C documentation.

Here's a simple helper function that will format JS dates for you.

function toIsoString(date) {
  var tzo = -date.getTimezoneOffset(),
      dif = tzo >= 0 ? '+' : '-',
      pad = function(num) {
          return (num < 10 ? '0' : '') + num;
      };

  return date.getFullYear() +
      '-' + pad(date.getMonth() + 1) +
      '-' + pad(date.getDate()) +
      'T' + pad(date.getHours()) +
      ':' + pad(date.getMinutes()) +
      ':' + pad(date.getSeconds()) +
      dif + pad(Math.floor(Math.abs(tzo) / 60)) +
      ':' + pad(Math.abs(tzo) % 60);
}

var dt = new Date();
console.log(toIsoString(dt));