Add days across Daylight Saving Time

When adding days to a time, it’s assuming each day is 24 hours long. This is not a correct assumption when crossing over daylight saving time.

Steps to recreate

import dayjs from 'dayjs';
import utc from 'dayjs/plugin/utc';
import tz from 'dayjs/plugin/timezone';

dayjs.extend(utc);
dayjs.extend(tz);

// 2020-10-25 is when DST starts in Europe/London (clocks go back by one hour). 
// This means that 2020-10-25 is a day that's only 23 hours long
const date1 = dayjs.tz('2020-10-24', 'Europe/London').add(2, 'day').toISOString();
const date2 = dayjs.tz('2020-10-26', 'Europe/London').toISOString();

// Prints
// {
//   date1: '2020-10-25T23:00:00.000Z',
//   date2: '2020-10-26T00:00:00.000Z'
// }
console.log({ date1, date2 });

Expected behavior date1 and date2 should be the same, as adding the number of “days” to the date should’ve allowed for the fact that not all days are 24 hours.

Information

• Day.js Version: v1.9.2
• OS: Windows 10 20H2
• Browser: Chromium: 87.0.4280.101
• Time zone: (UTC -04:00) Santiago

[ChristianKlima]

We completely switched our project to dayjs and at the end noticed that dayjs does not support the daylight saving time. I can only warn against using this library. Operations such as add cause unexpected results. As soon as a calculation goes over the limit of the time change, the result is wrong. We will continue to use moment.js. This library can only be used in countries without daylight saving time. Example:

dayjs(‘25-10-2020’, ‘DD-MM-YYYY’) .tz(‘Europe/Berlin’) .add(1, ‘day’) .diff(dayjs(‘26-10-2020’, ‘DD-MM-YYYY’).tz(‘Europe/Berlin’), ‘days’) .toString()

result = -3600000

[moulinraphael]

This is an important issue for us, but there is a workaround possible…

1. Get the date’s TZ
2. Add days using date method
3. Use tz method to “reset” TZ (date method doesn’t update the offset value)

export const addDaysExtended = (dateWithTZ, days) => {
  const clone = dayjs(dateWithTZ);
  const tz = clone.$x.$timezone;
  return clone.date(clone.date() + days).tz(tz, true);
};

const date1 = dayjs.tz('2020-10-24', 'Europe/London').add(2, 'day').toISOString();
const date2 = dayjs.tz('2020-10-26', 'Europe/London').toISOString();
const date3 = addDaysExtended(dayjs.tz('2020-10-24', 'Europe/London'), 2).toISOString();

console.log({ date1, date2, date3 });
// {
//    date1: '2020-10-25T23:00:00.000Z',
//    date2: '2020-10-26T00:00:00.000Z',
//    date3: '2020-10-26T00:00:00.000Z'
// }