https://javascript.info/date#setting-date-components

jst_dateAndTime_4_whichDayOfMonthWasManyDaysAgo.js

function getDateAgo(date, days) {
  const copiedDate = new Date(date);

  copiedDate.setDate(copiedDate.getDate() - days);

  return copiedDate;
}

let date = new Date(2015, 0, 2);

console.log( getDateAgo(date, 1).getDate() ); // 1, (1 Jan 2015)
console.log( getDateAgo(date, 2).getDate() ); // 31, (31 Dec 2014)
console.log( getDateAgo(date, 365).getDate() ); // 2, (2 Jan 2014)

console.log( getDateAgo(date, 1) );
console.log( getDateAgo(date, 2) );
console.log( getDateAgo(date, 365) );