Inconsistent determination of valid dates using Date.parse

If I try to parse a date with this syntax:

var date1 = new Date(Date.parse('2013' + '/' + '02' + '/' + '29'));

it will return 1 March 2013.

var date1 = new Date(Date.parse('2013' + '/' + '02' + '/' + '30'));

it will return 2 March 2013. But if I do

var date1 = new Date(Date.parse('2013' + '/' + '02' + '/' + '33'));

it will return Invalid Date.

My point is, why don't all of these dates return Invalid Date?