1118690 - new Date(1965, 0, 1).getFullYear() returns 1964 in the UK timezone

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jon Coppeard (:jonco)]

In the UK timezone, the following happens:

  js> new Date(1965, 0, 1).getFullYear();
  1964

The Date() constructor converts local time to UTC.  At 1st January 1965 the UK was on GMT, so this should return 1965.

As mentioned in bug 1093130, I think this is because to determine DST for years before 1970 we map them to 'equivalent' years, and we map 1965 to 1971 which falls inside a period where the UK was on GMT+1 due to the British Standard Time experiment (see http://en.wikipedia.org/wiki/British_Summer_Time#Periods_of_deviation for details).

I think the problem is the idea of mapping to equivalent years, since these years may not be equivalent depending on your location.

Comment 1

[André Bargull [:anba]]

Fixed in https://hg.mozilla.org/mozilla-central/rev/b7ef07909cc4


Before (Ubuntu - Time zone is Europe/London, current system time doesn't observe DST, e.g. set to "Jan 12 2017"):
new Date(1965, 0, 1).getFullYear()
1964


After (Ubuntu - Time zone is Europe/London, current system time doesn't observe DST, e.g. set to "Jan 12 2017"):
1965