Which day of month was many days ago?

Create a function getDateAgo(date, days) to return the day of month days ago from the date.

For instance, if today is 20th, then getDateAgo(new Date(), 1) should be 19th and getDateAgo(new Date(), 2) should be 18th.

Should work reliably for days=365 or more:

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

alert( getDateAgo(date, 1) ); // 1, (1 Jan 2015)
alert( getDateAgo(date, 2) ); // 31, (31 Dec 2014)
alert( getDateAgo(date, 365) ); // 2, (2 Jan 2014)
        

P.S. The function should not modify the given date.

Open a sandbox with tests.

The idea is simple: to substract given number of days from date:

          function getDateAgo(date, days) {
  date.setDate(date.getDate() - days);
  return date.getDate();
}
        

…But the function should not change date. That’s an important thing, because the outer code which gives us the date does not expect it to change.

To implement it let’s clone the date, like this:

function getDateAgo(date, days) {
  let dateCopy = new Date(date);

  dateCopy.setDate(date.getDate() - days);
  return dateCopy.getDate();
}

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

alert( getDateAgo(date, 1) ); // 1, (1 Jan 2015)
alert( getDateAgo(date, 2) ); // 31, (31 Dec 2014)
alert( getDateAgo(date, 365) ); // 2, (2 Jan 2014)

Open the solution with tests in a sandbox.