Incorrect Javascript Day When Added?

Question

I have a javascript function that takes in a number X and a date, and returns a new Date that is X number of days away:

function addDays(theDate, numDaysToAdd) {
    var newDate = new Date();
    return new Date(newDate.setDate(theDate.getDate() + numDaysToAdd));
}

I pass it a day that is Sat Jul 02 2016 16:03:06 GMT-0700 (Pacific Daylight Time) and a number 7, but the result I got was Thu Jun 09 2016 16:05:32 GMT-0700 (Pacific Daylight Time). Why is it giving me the correct date but wrong month?

p.s.w.g · Accepted Answer · 2016-06-27 23:21:49Z

1

The problem is that newDate is always created from the current date (new Date()). In other words, if this function is executed in June it will produce a date in June, then try to set a the day of the month as a offset from the input date.

You need to construct newDate as a copy of theDate:

function addDays(theDate, numDaysToAdd) {
    var newDate = new Date(theDate);
    newDate.setDate(theDate.getDate() + numDaysToAdd);
    return newDate;
}

var d = new Date('Sat Jul 02 2016 16:03:06 GMT-0700 (Pacific Daylight Time)');
console.log(addDays(d, 7).toString());

answered Jun 27, 2016 at 23:21

p.s.w.g

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Community · Accepted Answer · 2020-06-20 09:12:55Z

1

You can add number of milliseconds to given date and it will generate correct date.

getTime() returns milliseconds from epoch.

offset = numDaysToAdd * 24 * 60 * 60 * 1000;

24: Hours in a day

60: Minutes in an hour

60: seconds in a minute

1000: milliseconds in a second

Date constructor takes milliseconds from epoch

function addDays(theDate, numDaysToAdd) {
    var start = theDate.getTime();
    var offset = numDaysToAdd * 24 * 60 * 60 * 1000;
    return new Date(start + offset);
}

var today = new Date();
console.log(today, addDays(today, 10));

edited Jun 20, 2020 at 9:12

CommunityBot

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answered Jun 27, 2016 at 23:14

Zohaib Ijaz

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