Add days to a date without changing GMT time

Question

I am working on an application where all dates used are round GMT dates, e.g. 2015-10-29T00:00:00.000Z.

I am using following function to add days to a date :

function addDays(date, days) {
  var result = new Date(date);
  result.setDate(result.getDate() + days);
  return result;
}

But, I just realized it doesn't work when crossing daylight saving time changing day :

var myDate = new Date('2015-10-24T00:00:00.000Z');
for(i = 0; i<4; i++) {
  console.log(JSON.stringify(myDate));
  myDate = addDays(myDate, 1);
}

Outputs this :

"2015-10-24T00:00:00.000Z"
"2015-10-25T00:00:00.000Z"
"2015-10-26T01:00:00.000Z"
             ^
"2015-10-27T01:00:00.000Z"
             ^

Note that the two last dates are not round anymore.

What is the proper way to deal with this ?

Answer 1

Edit: Actually I found a solution that is definitely better. I'm leaving this for the sake of exhaustivity.

The problem is that Date.setDate() adds a day without changing the local time. But this means the GMT time does change !

Adding a day without changing the GMT time is actually as simple as adding 24 * 3600 * 1000 milliseconds to the date's time :

function addGMTDays(date, days) {
  var result = new Date(date);
  result.setTime(result.getTime() + days * 24 * 3600 * 1000);
  return result;
}

Then :

var myDate = new Date('2015-10-24T00:00:00.000Z');
for(i = 0; i<4; i++) {
  console.log(JSON.stringify(myDate));
  myDate = addGMTDays(myDate, 1);
}

Outputs this :

"2015-10-24T00:00:00.000Z"
"2015-10-25T00:00:00.000Z"
"2015-10-26T00:00:00.000Z"
"2015-10-27T00:00:00.000Z"