Date

var d1 = new Date(2015, 4, 31)
var d2 = new Date(2015, 4, 32)
var d3 = new Date(2015, 4, -1)
console.log(d1)
console.log(d2)
console.log(d3)
console.log(d1.getDate(), d1.getMonth() + 1)
console.log(d2.getDate(), d2.getMonth() + 1)
console.log(d3.getDate(), d3.getMonth() + 1)
/*
 Sun May 31 2015 00:00:00 GMT+0800 (中国标准时间)
 Mon Jun 01 2015 00:00:00 GMT+0800 (中国标准时间)
 Wed Apr 29 2015 00:00:00 GMT+0800 (中国标准时间)
 31 5
 1 6
 29 4
*/

var d = new Date(2015, -1)
var year = d.getFullYear()
var month = d.getMonth() + 1
var day = d.getDate()
console.log(year, month, day) // 2014 12 1