Why does Date.parse give incorrect results?

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)

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Why is the second parse incorrect?

Answer 1

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?). 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 and Date#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
}