Daily times are wrong around DST when system is not UTC

Question

There are a number of much older issues around this (#65 , #157 , #213) and I was hoping it would be resolved with a fix for #38 et al., and a fix (without a test) was even merged in for it (#232), but it continues to be an issue.

Repro:

es6
const rrule = RRule.fromString('DTSTART=20181101T100000Z;UNTIL=20181106T100000Z;FREQ=DAILY')
expect(rrule.all()).to.deep.equal([
  new Date('2018-11-01T10:00:00.000Z'),
  new Date('2018-11-02T10:00:00.000Z'),
  new Date('2018-11-03T10:00:00.000Z'),
  new Date('2018-11-04T10:00:00.000Z'),
  new Date('2018-11-05T10:00:00.000Z'),
  new Date('2018-11-06T10:00:00.000Z')
])

Result:

      AssertionError: expected [ Array(5) ] to deeply equal [ Array(6) ]
      + expected - actual

       [
         [Date: 2018-11-01T10:00:00.000Z]
         [Date: 2018-11-02T10:00:00.000Z]
         [Date: 2018-11-03T10:00:00.000Z]
      -  [Date: 2018-11-04T11:00:00.000Z]
      -  [Date: 2018-11-05T11:00:00.000Z]
      +  [Date: 2018-11-04T10:00:00.000Z]
      +  [Date: 2018-11-05T10:00:00.000Z]
      +  [Date: 2018-11-06T10:00:00.000Z]
       ]

The resulting times for a daily repeating routine are off by 1 hour after "fall back". This only reproduces when the machine's time zone is one with DST, so it does not exist under UTC, for example.

该提问来源于开源项目：jakubroztocil/rrule

Answer 1

This is because timeset is setting the time to be a consistent time (which is the daily routine's time in UTC), but the JavaScript Date constructor is then applying the local timezone offset (here, changing from PDT to PST):

es6
> new Date(2018, 10, 3, 4, 0, 0, 0)
2018-11-03T11:00:00.000Z
> new Date(2018, 10, 4, 4, 0, 0, 0)
2018-11-04T12:00:00.000Z

The combine function should not naively combine dates and times, but rather needs to adjust the time for the appropriate timezone offset on that date.