Answers for "why i have to subtract month when work with date in javascript"

minus month javascript date

var d = new Date("January 14, 2012");
console.log(d.toLocaleDateString());
d.setMonth(d.getMonth() - 3);
console.log(d.toLocaleDateString());

javascript subtract years from date

var date = new Date(); 
date.setDate(date.getDate() + days);
date.setMonth(date.getMonth() + months);
date.setFullYear(date.getFullYear() + years);
console.log((date.getMonth() ) + '/' + (date.getDate()) + '/' + (date.getFullYear()));