Javascript: parse a string to Date as LOCAL time zone

Solution 1:

Parsing of date strings using the Date constructor or Date.parse (which are essentially the same thing) is strongly recommended against.

If Date is called as a function and passed an ISO 8601 format date string without a timezone (such as 2015-11-24T19:40:00), you may get one of the following results:

1. Pre ES5 implementaitons may treat it as anything, even NaN (such as IE 8)
2. ES5 compliant implementations will treat it as UTC timezone
3. ECMAScript 2015 compliant implementations will treat it as local (which is consistent with ISO 8601)

A Date object has a time value which is UTC, and an offset based on system settings. When you send a Date to output, what you see is usually the result of Date.prototype.toString, which is an implementation dependent, human readable string representing the date and time, usually in a timezone based on system settings.

The best way to parse date strings is to do it manually. If you are assured that the format is consistent and valid, then parsing an ISO format string as a local date is as simple as:

/*  @param {string} s - an ISO 8001 format date and time string
**                      with all components, e.g. 2015-11-24T19:40:00
**  @returns {Date} - Date instance from parsing the string. May be NaN.
*/
function parseISOLocal(s) {
  var b = s.split(/\D/);
  return new Date(b[0], b[1]-1, b[2], b[3], b[4], b[5]);
}

document.write(parseISOLocal('2015-11-24T19:40:00'));

Note that parsing of ISO strings using Date.parse only accepts UTC, it does not accept any other timezone designation (noting the above behaviour if it's missing).

Solution 2:

A variation on RobG's terrific answer.

Note that this will require that you run bleeding edge JavaScript as it relies on the arrow notation and spread operator.

function parseDateISOString(s) {
  let ds = s.split(/\D/).map(s => parseInt(s));
  ds[1] = ds[1] - 1; // adjust month
  return new Date(...ds);
}

Note that this doesn't take into account if the date/time given is in any timezone. It will assume local time. You can change new Date for Date.UTC to assume UTC.

There are technical reasons for why you would write you code like this. For example, here we apply the correct number of arguments, with their corresponding expected type. It's true that the Date constructor will turn strings into numbers but what could be happening is that there's a deoptimization taking place where the optimized code is expecting a number but sees a string and takes a slower path. Not a big deal but I try to write my JavaScript to avoid such things. We also won't be indexing outside the bounds of the array if less than 6 components can be found in the string which is also one of those things you can do in JavaScript but it has subtle deoptimization caveats.