Doesn't add day

Version:1.7.5 Wasn’t reproducible on version 1.6.x (don’t remember now) Similar issue to: https://github.com/iamkun/dayjs/issues/262

Trying to add day to 28 of October 2018 doesn’t add anything: https://codepen.io/lexswed/pen/NLaRZY

console.log(dayjs('2018-10-28').format('MMM dd DD'))
console.log(dayjs('2018-10-28').add(1, 'day').format('MMM dd DD'))

[iamkun]

@nukuuk @LexSwed You may could try v1.7.7 and check if this issue is fixed in your timezone. THX.

[prantlf]

@LexSwed, you need to consider the fact, that dayjs works in the local time zone by default, just like Date. There are PRs #168 and #326 to introduce a “UTC mode” which will allow work with an apparent “date-only” value, which you use in your case.

Your case is similar to #262, #249 and #74.

When you constructed the dayjs instance like this:

dayjs('2018-10-28')

you assumed, that it works like a “date-only” instance. After adding 1 day, you expected it to become like dayjs('2018-10-28'). However, it was wrong. Both dayjs and Date assumed, that you supplied a string in UTC and you just omitted the time part. You actually performed the following initialization:

dayjs('2018-10-28T00:00:00Z')

It means, that you have a full date+time object. Both dayjs and Date work in the local time zone by default, which you can confirm by using getters, setters and toString. The actually stored value will be this in Central Europe:

2018-10-28 01:00:00 GMT+01:00 (CET)

Depending on DST changes in your local time zone, adding or subtracting a day may result in an hour shift, which may affect your day value. If you want to prevent it, you need to use setters with UTC in their name, when working with Date. When working with dayjs you can do nothing, because there are no UTC-setters there yet. You workaround will work, or you can try the UTC mode from the PRs.