phantomjs parses dates incorrectly for dates before March in the year of 1400, why? - phantomjs

Question

Is it some sort of overflow?
phantomjs> new Date("1400-03-01T00:00:00.000Z")
"1400-03-01T00:00:00.000Z"
phantomjs> new Date("1400-02-28T20:59:59.000Z")
"1400-02-27T20:59:59.000Z"
what you would expect:
>>(new Date("1400-03-01T00:00:00.000Z")).toISOString()
"1400-03-01T00:00:00.000Z"
>>(new Date("1400-02-28T20:59:59.000Z")).toISOString()
"1400-02-28T20:59:59.000Z"
apparently there is a gap of 24 hours when parsing dates between the 28th of February in 1400 and the 1st of March in 1400.
any ideas?
Phantomjs anyway is obsolete but still ... our legacy tests are failing when we try to upgrade to chrome headless ...

Answer 1

PhantomJS uses a version of Qt WebKit which is maintained independently of Qt.
The date format you are using is part of the ISO-8601 date and time format. [related]
The version of Qt WebKit that PhantomJS uses has a function that parses dates of the form defined in ECMA-262-5, section 15.9.1.15 (similar to RFC 3339 / ISO 8601: YYYY-MM-DDTHH:mm:ss[.sss]Z).
In the source code, we can see that the function used to parse these types of dates is called:
double parseES5DateFromNullTerminatedCharacters(const char* dateString)
The file that contains this function in the PhantomJS repository has not been updated since July 27, 2014, while the official file was updated as recently as October 13, 2017.
It appears that there is a problem in the logic having to do with handling leap years.
Here is a comparison of DateMath.cpp between the most recent versions from the official qtwebkit repository (left) and the PhantomJS qtwebkit repository (right).