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.