Calling moment.utc("2016-07-01").year(2013).toString() provides the wrong date

[stringbeans]

Description of the Issue and Steps to Reproduce:

It seems like when setting the date to a date in the past and then setting the year to anything will push the outputted date to be at the end of the month.

If you call moment.utc("2016-07-01").year(2013).toString() you will get Wed Jul 31 2013 00:00:00 GMT+0000. Notice how it is Jul 31 and not Jul 01.

Environment:

I've reproduced this in Chrome 61 as well as in Node 5.10.1

Other information that may be helpful:

This seems to only be an issue if you are using a date from the year 2016. If you try moment.utc("2015-07-01").year(2013).toString() it works fine.

I can confirm that this is not an issue previous to moment 2.19.0+. It only is an issue from 2.19.0+

If you are reporting an issue, please run the following code in the environment you are using and include the output:

console.log( (new Date()).toString())
console.log((new Date()).toLocaleString())
console.log( (new Date()).getTimezoneOffset())
console.log( navigator.userAgent)
console.log(moment.version)

Thu Oct 12 2017 23:10:36 GMT-0300 (ADT)
10/12/2017, 11:10:36 PM
180
Mozilla/5.0 (Macintosh; Intel Mac OS X 10_12_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36
2.19.1

[HolgerFrank]

In version 2.19.0 the function set$1() was changed.

Because of this change, setting a new year for a leap year moment, it always sets the last day of the month as date.

moment('2010-02-25').year(2017) // result is: 2017-02-28
moment('2010-10-15').year(2017) // result is: 2017-10-31

Current (incorrect) implementation

function set$1 (mom, unit, value) {
    if (mom.isValid() && !isNaN(value)) {
        if (unit === 'FullYear' && isLeapYear(mom.year())) {
            mom._d['set' + (mom._isUTC ? 'UTC' : '') + unit](value, mom.month(), daysInMonth(value, mom.month()));
        }
        else {
            mom._d['set' + (mom._isUTC ? 'UTC' : '') + unit](value);
        }
    }
}

Because the special case is only needed for th 29th of Februar in leap years, the correct implementation should be:

function set$1 (mom, unit, value) {
    if (mom.isValid() && !isNaN(value)) {
        if (unit === 'FullYear' && isLeapYear(mom.year()) && mom.month() === 1 && mom.date() === 29) {
            mom._d['set' + (mom._isUTC ? 'UTC' : '') + unit](value, 1, daysInMonth(value, 1));
        }
        else {
            mom._d['set' + (mom._isUTC ? 'UTC' : '') + unit](value);
        }
    }
}

Because this is a critical error it should be corrected as soon as possible.

[stringbeans]

sorry just noticed i wrote "I can confirm that this is not an issue previous to moment 1.19.0+. It only is an issue from 1.19.0+" but I actually meant 2.19... ive updated this in the original post

[Kerumen]

@HolgerFrank Would you mind submit a PR with your change to fix this? It's a pretty critical bug which should be fixed asap..

Edit: I submitted the PR, couldn't wait.

[maggiepint]

Closing in favor of PR.