Why does Date.parse give incorrect results?

ghz 1years ago ⋅ 428 views

Question

Case One:

new Date(Date.parse("Jul 8, 2005"));

Output:

Fri Jul 08 2005 00:00:00 GMT-0700 (PST)

Case Two:

new Date(Date.parse("2005-07-08"));

Output:

Thu Jul 07 2005 17:00:00 GMT-0700 (PST)

Why is the second parse incorrect?

Answer

Until the 5th edition spec came out, the Date.parse method was completely implementation dependent (new Date(string) is equivalent to Date.parse(string) except the latter returns a number rather than a Date). In the 5th edition spec the requirement was added to support a simplified (and slightly incorrect) ISO-8601 (also see [What are valid Date Time Strings in JavaScript?](https://stackoverflow.com/questions/51715259/what-are-valid-date- time-strings-in-javascript/)). But other than that, there was no requirement for what Date.parse / new Date(string) should accept other than that they had to accept whatever Date#toString output (without saying what that was).

As of ECMAScript 2017 (edition 8), implementations were required to parse their output for Date#toString and Date#toUTCString, but the format of those strings was not specified.

As of ECMAScript 2019 (edition 9) the format for [Date#toString](http://ecma-international.org/ecma-262/9.0/#sec- date.prototype.tostring) and [Date#toUTCString](http://ecma- international.org/ecma-262/9.0/#sec-date.prototype.toutcstring), have been specified as (respectively):

  1. ddd MMM DD YYYY HH:mm:ss ZZ [(timezone name)]
    e.g. Tue Jul 10 2018 18:39:58 GMT+0530 (IST)

  2. ddd, DD MMM YYYY HH:mm:ss Z
    e.g. Tue 10 Jul 2018 13:09:58 GMT

providing 2 more formats that Date.parse should parse reliably in new implementations (noting that support is not ubiquitous and non–compliant implementations will remain in use for some time).

I would recommend that date strings are parsed manually and the Date constructor used with year, month and day arguments to avoid ambiguity:

// parse a date in yyyy-mm-dd format
function parseDate(input) {

  let parts = input.split('-');

  // new Date(year, month [, day [, hours[, minutes[, seconds[, ms]]]]])
  return new Date(parts[0], parts[1]-1, parts[2]); // Note: months are 0-based
}
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