Date.toISOString() but local time instead of UTC

Question

Let's say we have this datetime:

var d = new Date("Sat Jul 21 2018 14:00:00 GMT+0200");

Exporting it as a string (console.log(d)) gives inconsistent results among browsers:

• Sat Jul 21 2018 14:00:00 GMT+0200 (Paris, Madrid (heure d’été)) with Chrome

• Sat Jul 21 14:00:00 UTC+0200 2018 with Internet Explorer, etc.

so we can't send datetime to a server with an unconsistent format.

The natural idea then would be to ask for an ISO8601 datetime, and use d.toISOString(); but it gives the UTC datetime: 2018-07-21T12:00:00.000Z whereas I would like the local-timezone time instead:

2018-07-21T14:00:00+0200
or
2018-07-21T14:00:00

How to get this (without relying on a third party dependency like momentjs)?

I tried this, which seems to work, but isn't there a more natural way to do it?

var pad = function(i) { return (i < 10) ? '0' + i : i; };

var d = new Date("Sat Jul 21 2018 14:00:00 GMT+0200");
Y = d.getFullYear();
m = d.getMonth() + 1;
D = d.getDate();
H = d.getHours();
M = d.getMinutes();
S = d.getSeconds();
s = Y + '-' +  pad(m) + '-' + pad(D) + 'T' + pad(H) + ':' + pad(M) + ':' + pad(S);
console.log(s);

Answer 1

There is limited built-in support for formatting date strings with timezones in ECMA-262, there is either implementation dependent toString and toLocaleString methods or toISOString, which is always UTC. It would be good if toISOString allowed a parameter to specify UTC or local offset (where the default is UTC).

Writing your own function to generate an ISO 8601 compliant timestamp with local offset isn't difficult:

function toISOLocal(d) {
  var z  = n =>  ('0' + n).slice(-2);
  var zz = n => ('00' + n).slice(-3);
  var off = d.getTimezoneOffset();
  var sign = off > 0? '-' : '+';
  off = Math.abs(off);

  return d.getFullYear() + '-'
         + z(d.getMonth()+1) + '-' +
         z(d.getDate()) + 'T' +
         z(d.getHours()) + ':'  + 
         z(d.getMinutes()) + ':' +
         z(d.getSeconds()) + '.' +
         zz(d.getMilliseconds()) +
         sign + z(off/60|0) + ':' + z(off%60); 
}

console.log(toISOLocal(new Date()));

Answer 2

The trick is to adjust the time by the timezone, and then use toISOString(). You can do this by creating a new date with the original time and subtracting by the timezone offssetfrom the original time:

var d = new Date("Sat Jul 21 2018 14:00:00 GMT+0200");
var newd = new Date(d.getTime() - d.getTimezoneOffset()*60000);
console.log(newd.toISOString())

Alternatively, you can simply adjust the original date:

var d = new Date("Sat Jul 21 2018 14:00:00 GMT+0200");
d = new Date(d.getTime() - d.getTimezoneOffset()*60000);
console.log(d.toISOString())

For your convenience, the result from .getTime() is the number of milliseconds since 1 January 1970. However, getTimezoneOffset() gives a time zone difference from UTC in minutes; that’s why you need to multiply by 60000 to get this in milliseconds.

Of course, the new time is still relative to UTC, so you’ll have to ignore the Z at the end:

d = d.slice(0,-1);

Answer 3

My version:

// https://stackoverflow.com/questions/10830357/javascript-toisostring-ignores-timezone-offset/37661393#37661393
// https://stackoverflow.com/questions/49330139/date-toisostring-but-local-time-instead-of-utc/49332027#49332027
function toISOLocal(d) {
  const z = n => ('0' + n).slice(-2);
  let off = d.getTimezoneOffset();
  const sign = off < 0 ? '+' : '-';
  off = Math.abs(off);
  return new Date(d.getTime() - (d.getTimezoneOffset() * 60000)).toISOString().slice(0, -1) + sign + z(off / 60 | 0) + ':' + z(off % 60);
}
console.log(toISOLocal(new Date()));