JS - Calculate number of days between 2 dates considering leap year

Question

I want to calculate the number of days between 2 dates. Common problem.

For exemple :

var d0 = new Date("2016-02-27");
var d1 = new Date("2017-08-25");

Many people recommend using epoch difference :

var res = (d1 - d0) / 1000 / 60 / 60 / 24;
// res = 545 days

But I was doubtful so I wrote a naive function :

function days(d0, d1)
{
    var d = new Date(d0);
    var n = 0;
    while(d < d1)
    {
        d.setDate(d.getDate() + 1);
        n++;
    }
    return n;
}

This function and epoch difference essentially output the same result but not with my particuliar exemple. Probably because 2016 is a leap year.

res = days(d0, d1);
// res = 546 days

Any idea why ?

Answer 1

After testing, it appears that the loop stop at 2017-08-26, instead of 2017-08-25.

When you print the value of d0 and d1, here is the result :

d0: Sat Feb 27 2016 01:00:00 GMT+0100 (Central Europe Standard Time)

d1: Fri Aug 25 2017 02:00:00 GMT+0200 (Central Europe Daylight Time)

As you can see, there is a one-hour shift between the two dates, so when the index of the loop reaches Aug 25 2017, this shift is still there and makes the "lower than" operation true, where it should be false.

Make sure to normalize your dates before using it.