FOO FOO - 2 months ago 47
Javascript Question

JavaScript Date.prototype.toISOString() loses offset

Why does this method use UTC timezone (

Z
) and not include the local time offset (
+/-HH:SS
) instead? The "ISO" in the method name refers to ISO 8601—which allows for "time zone designations" to be expressed as part of its format.

In other words,
new Date()
tells me both the date and time, and the timezone offset (via
getTimezoneOffset()
). But
toISOString()
only tells me the date and time in one timezone—it discards the information of what time it was in the locale that the
new Date()
originated.

Wouldn't it make sense for
toISOString()
to also include the originating timezone's offset from UTC?
toISOString()
's omission of +/-HH:SS loses information about the originating
Date
if it's used for serializing.

All my AJAX calls (Angular, jQuery) serialize via
toISOString()
, thus losing the local time of the serialized date when it's communicated to the server. Any way to get a JavaScript
Date
to output an ISO-formatted string that also includes offset (besides using a library like Moment.js), or do I need to write my own method?

Answer

This is one of those "because that's what the language specification says" answers (see ECMA-262 ยง20.3.4.36). ISO 8601 is a format, and while it allows the use of timezone data, ECMAScript only uses UTC. You can extend Date.prototype with your own toLocalISOString method if you wish. BTW, writing such a method is not difficult.

// Format date as ISO 8601 long format with local timezone offset
if (!Date.prototype.toLocalISOString) {
  Date.prototype.toLocalISOString = function() {
  
  // Helper for padding
  function pad(n, len) {
    return ('000' + n).slice(-len);
  }

  // If not called on a Date instance, or timevalue is NaN, return undefined
  if (isNaN(this) || Object.prototype.toString.call(this) != '[object Date]') return;

  // Otherwise, return an ISO format string with the current system timezone offset
  var d = this;
  var os = d.getTimezoneOffset();
  var sign = (os > 0? '-' : '+');
  os = Math.abs(os);

  return pad(d.getFullYear(), 4) + '-' +
         pad(d.getMonth() + 1, 2) + '-' +
         pad(d.getDate(), 2) +
         'T' + 
         pad(d.getHours(), 2) + ':' +
         pad(d.getMinutes(), 2) + ':' +
         pad(d.getSeconds(), 2) + '.' +
         pad(d.getMilliseconds(), 3) + 
       
         // Note sign of ECMASCript offsets are opposite to ISO 8601
         sign +
         pad(os/60 | 0, 2) + ':' +
         pad(os%60, 2);
  }
}
document.write(new Date().toLocalISOString())

Edit

Based on a post by DanDascalescu, here's an alternative that might be more efficient as it has fewer function calls, but it creates two additional Date objects:

// Return a string in ISO 8601 extended format with the host timezone offset
Date.prototype.toLocalISOString = function() {

    // If not called on a Date instance, or timevalue is NaN, return undefined
    if (isNaN(this) || Object.prototype.toString.call(this) != '[object Date]') return;

    // Copy date so don't modify original
    var d = new Date(+this);
    var offset = d.getTimezoneOffset();
    var offSign = offset > 0? '-' : '+';
    offset = Math.abs(offset);
    var tz = offSign + ('0' + (offset/60|0)).slice(-2) + ':' + ('0' + offset%60).slice(-2)
    return new Date(d.setMinutes(d.getMinutes() - d.getTimezoneOffset())).toISOString().slice(0,-1) + tz; 
}

console.log(new Date().toLocalISOString())

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