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var d = new Date();
d.setDate(0); //sets d to the last day of the previous month
d.setDate(1); //sets d the the first day of that month
d.setHours(0,0,0,0); //sets d time to midnight
//d now equals the first day of the month before today
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var date = new Date(), y = date.getFullYear(), m = date.getMonth();
var firstDay = new Date(y, m, 1);
var lastDay = new Date(y, m + 1, 0);
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var lastday = function(y,m){
return new Date(y, m +1, 0).getDate();
}
console.log(lastday(2014,0));
console.log(lastday(2014,1));
console.log(lastday(2014,11));