考虑到时区变化，通过 date-fns 添加一天

Question

In my project I use date-fns for date manipulations. There is a need to iterate days in some range. For that I'm using following code:

  for (
    // from and to are always start of some day, from <= to
    let date = from;
    isBefore(date, to) || isEqual(date, to);
    date = addDays(date, 1)
  ) {
    // Some operations with date
  }

I'm expecting date to always be the start of some day, but in case timezone changes (winter time -> summer time), date is 1 hour less then expected. Here is an example:

const from = new Date('2019-03-31T00:00:00.000Z')
const fromPlusDay = dateFns.addDays(from, 1)

// I'm getting "2019-03-31T23:00:00.000Z"
// instead of "2019-04-01T00:00:00.000Z"
fromPlusDay.toISOString()

By the way my time zone was +2 and after moving to summer time it became +3

Answer 1

I faced with the same problem:

6 October 2019 in Australia is Daylight Saving Time Starts.

dh.parse('2019-10-06')

returns: Sat Oct 05 2019 23:00:00 GMT+1000 (Australian Eastern Standard Time)

Solution is adding info about time zone (one of):

1. Add sign of absent time zone offset “Z” -- dh.parse('2019-10-06T00:00:00.000Z' )
2. Add GMT -- dh.parse('2019-10-06 GMT' )
3. Add +00:00 -- dh.parse('2019-10-06T00:00:00.000+00:00' )

] 1 )

Answer 2

It is considering your time zone change and it i s at the start of day (in your timezone, not UTC). Midnight in UTC is not necessarily start of day in your zone.

Check this out (I'm in GMT+1 zone):

const from = dateFns.parse('2019-03-31') // -> "2019-03-31T00:00:00.000+0100"
const fromPlusDay = dateFns.addDays(from, 1)

dateFns.format(fromPlusDay , 'YYYY-MM-DDTHH:mm:ss.SSSZZ') // -> "2019-04-01T00:00:00.000+0200"

I think what you are doing is fine, just don't expect your zone to be 00:00 in UTC, and avoid printing dates in UTC.