Adding a month to a date removes an hour.

[dennisholmer]

Describe the bug
Adding a month to a date removes an hour.

Expected behavior

const initialDate = dayjs('2022-02-28T00:00:00.000Z');
  const oneMonthAdded = initialDate.add(1, 'month');
  console.log(oneMonthAdded.toISOString());

Should output 2022-03-28T00:00:00.000Z

Actual behaviour
Outputs 2022-03-27T23:00:00.000Z

Information

• Day.js Version: 1.10.7
• OS: Win 10
• Browser: N/A
• Time zone: GMT+2

[mp3000mp]

Hi, I think this is the expected behaviour.

Some timezone does not have the same UTC all year.

For example in France we're UTC+2 from March to Octobre and UTC+1 from Novembre to February.

You can see that the behavior is the same with Day.js, Moment.js and native Date().
Please note that you can use the timezone plugin in order to take into account the time change when you format your Day.js object.

// dayjs
const d = dayjs('2022-02-28T00:00:00.000Z')
const d2 = d.add(1, 'month')
// moment
const m = moment('2022-02-28T00:00:00.000Z')
const m2 = moment('2022-02-28T00:00:00.000Z')
m2.add(1, 'month')
// native
const j = new Date(2022, 1, 28, 0, 0, 0)
const j2 = new Date(2022, 1, 28, 0, 0, 0)
j2.setMonth(j2.getMonth() + 1)

// one hour difference in relation to UTC
console.log(d.toISOString())
console.log(d2.toISOString())
// same behavior with moment
console.log(m.toISOString())
console.log(m2.toISOString())
// same behavior with native js Date object
console.log(j.toISOString())
console.log(j2.toISOString())

// we can see the explanation here because the utcOffset is different
console.log(d.utcOffset())
console.log(d2.utcOffset())
console.log(j.getTimezoneOffset())
console.log(j2.getTimezoneOffset())

// the timezone plugin allows us to display the date in a consistant way regarding to the user timezone
console.log(d.tz('Europe/Paris').format())
console.log(d2.tz('Europe/Paris').format())