First of all, I think timezone probably has something to do with this. I'm in EST/EDT. Also, I'm testing this on chromium 17 / linux.

Now, let's say I create two dates like this:

// December 5

dateFromNumbers = new Date(2020, 11, 5);
dateFromString = new Date("2020-12-5");

It seems these dates should have identical timestamps, and they do:

+dateFromNumbers == +dateFromString; // true

...at least in this case. But in some cases, they don't:

// December 15

dateFromNumbers = new Date(2020, 11, 15);
dateFromString = new Date("2020-12-15");

+dateFromNumbers == +dateFromString; // false

What's going on here?

dateFromNumbers; // Tue Dec 15 2020 00:00:00 GMT-0500 (EST)
dateFromString;  // Mon Dec 14 2020 19:00:00 GMT-0500 (EST)

Looks like dateFromString is 5 hours earlier than dateFromNumbers in this case (EST is GMT - 5, I'm sure it's related somehow).

It seems to affect the ends of months October through December. Here's a fiddle that makes it easy to see which days differ (unless you are red-green colorblind, in that case it may be difficult to see, my apologies).

http://jsfiddle.net/9gBfX/

What gives?

---

Notes:

• You can set your system timezone to EST/EDT to see the jsfiddle example as I'm seeing it.
• Date's month numbers are zero-based; the 11 is not a typo.
• This issue appears in every year that I checked.

After looking in V8's source code:

// Specification:
// Accept ES5 ISO 8601 date-time-strings or legacy dates compatible
// with Safari.
<...>
//  A string that matches both formats (e.g. 1970-01-01) will be
//  parsed as an ES5 date-time string - which means it will default
//  to UTC time-zone. That's unavoidable if following the ES5
//  specification.

Reading the surrounding code, it seems that a date-time string with both month and day 2 symbols long is considered a valid ES5 date-time string. Further down in the ES5 parser, after it parses the dates and times, there's a comment:

// Successfully parsed ES5 Date Time String. Default to UTC if no TZ given.

In case of "YYYY-MM-DD", once the code gets that far, the ES5 parser has successfully parsed the entire string, so it returns before the legacy parser gets a chance to localize the timezone. Otherwise (month/day are one symbol long) it's treated as a "legacy" datetime and the legacy parser gets to deal with it and localize it.