Javascript Date issue when using setMonth

Question

When developing an firefox add-on the following gives different results?

var out = document.getElementById('out');

out.textContent += new Date(2015, 1, 6, 16, 0, 0, 0) + '\n'; // -> Date "2015-02-06T15:00:00.000Z" correct, months are zero-based
var dt = new Date(0, 0, 0, 0, 0, 0, 0);
dt.setFullYear(2015);
dt.setMonth(1);
dt.setDate(6);
dt.setHours(16);
out.textContent += dt + '\n'; // -> Date "2015-03-06T15:00:00.000Z" ??

<pre id="out"></pre>

The problem is the setMonth(1) sets March in the second case. This happens to arbitrary dates, other dates work just fine with both approaches. Any idea why?

var out = document.getElementById('out');

out.textContent += new Date(2015, 0, 30, 16, 0, 0, 0) + '\n'; // -> 2015-01-30T15:00:00.000Z" months are zero-based
var dt = new Date(0, 0, 0, 0, 0, 0, 0);
dt.setFullYear(2015);
dt.setMonth(0);
dt.setDate(30);
dt.setHours(16);
out.textContent += dt + '\n'; // -> 2015-01-30T15:00:00.000Z

<pre id="out"></pre>

Answer 1

It's because of wrapping, and has nothing to do with Firefox or Firefox addons.

var dt = new Date(0, 0, 0, 0, 0, 0, 0);
dt.setFullYear(2015);
dt.setMonth(1);
dt.setDate(6);
dt.setHours(16);

Originally, dt is Dec 31 1899.

Now, we set it to 2015: Dec 31 2015

The month becomes February, but because February only has 28 days, it wraps around to March: Mar 03 2015

And then, of course, the date is the sixth, so March 6th.