I haven't verified it by any means, but this to me smells like ES5 15.9.1.8[1]. Basically, prior to ES6, Javascript engines did not prescribe the use of IANA (nee Olson) tzdata to resolve historic dates.
The standard says: "The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round."
In Spidermonkey, at least, it mapped current years onto prior years where the current day falls on the same day of the week.
The standard says: "The implementation of ECMAScript should not try to determine whether the exact time was subject to daylight saving time, but just whether daylight saving time would have been in effect if the current daylight saving time algorithm had been used at the time. This avoids complications such as taking into account the years that the locale observed daylight saving time year round."
In Spidermonkey, at least, it mapped current years onto prior years where the current day falls on the same day of the week.
edit: Spidermonkey bug I filed a long time ago: https://bugzilla.mozilla.org/show_bug.cgi?id=1029923
[1]: https://es5.github.io/#x15.9.1.8