1342853 - getTimezoneOffset() returns incorrect values in Brasilia time zone

Status: RESOLVED DUPLICATE of bug 1346211

(Core :: JavaScript Engine, defect, P3)

Description

[Masashi Miyazaki]

User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101 Firefox/52.0
Build ID: 20170223185858

Steps to reproduce:

In Brasilia time zone, offset is 3 hours. The API getTimezoneOffset() returns 180. The Daylight Saving Time starts on the third Sunday in October and ends on the third Sunday in February. During DST, the time zone offset is 2 hours and getTmezoneOffset() returns 120. Below JavaScript code checks the time zone offset around DST boundary dates in Brasilia time zone.

(new Date(2017,9,14,12,0,0)).getTimezoneOffset() --> returns 180
(new Date(2017,9,15,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2018,1,17,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2018,1,18,12,0,0)).getTimezoneOffset() --> returns 180


Actual results:

getTimezoneOffset() returns incorrect value around October 21, 2018 when DST starts in Brasilia time zone.

(new Date(2018,9,20,12,0,0)).getTimezoneOffset() --> returns 120 which is incorrect, should return 180
(new Date(2018,9,21,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2019,1,16,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2019,1,17,12,0,0)).getTimezoneOffset() --> returns 180

It also returns incorrect values around October 20, 2019 and October 18, 2020.

(new Date(2019,9,19,12,0,0)).getTimezoneOffset() --> returns 120 which is incorrect, should return 180
(new Date(2019,9,20,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2020,1,15,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2020,1,16,12,0,0)).getTimezoneOffset() --> returns 180

(new Date(2020,9,17,12,0,0)).getTimezoneOffset() --> returns 120 which is incorrect, should return 180
(new Date(2020,9,18,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2021,1,20,12,0,0)).getTimezoneOffset() --> returns 120
(new Date(2021,1,21,12,0,0)).getTimezoneOffset() --> returns 180


Expected results:

(new Date(2018,9,20,12,0,0)).getTimezoneOffset() --> should return 180
(new Date(2019,9,19,12,0,0)).getTimezoneOffset() --> should return 180
(new Date(2020,9,17,12,0,0)).getTimezoneOffset() --> should return 180

Comment 1

[Sebastian Hengst [:aryx] (needinfo me if it's about an intermittent or backout)]

Expected result on Xubuntu. Issue is similar to bug 1330307.

Comment 2

[André Bargull [:anba]]

Dup'ing forward to bug 1346211 which should have fixed this problem. Please reopen if the issue is still present. Thanks!