how to get time after a month from now js code example

Example 1: javascript get day of year from date

//Month -> 0= Jan, 11=Dec | myDate = 6 Dec 2019 10:45
var myDate = new Date(2019, 11, 06, 10, 45);
var dayInYear = Math.floor((myDate - new Date(myDate.getFullYear(), 0, 0)) / (1000 * 60 * 60 * 24));

Example 2: javascript date 3 months ago

var someDate = new Date(); // add arguments as needed
someDate.setTime(someDate.getTime() - 3*28*24*60*60);
// assumes the definition of "one month" to be "four weeks".