This is just regular JavaScript, no need for TypeScript.

CopyyourDate = new Date(yourDate.getTime() + (1000 * 60 * 60 * 24));

1000 milliseconds in a second * 60 seconds in a minute * 60 minutes in an hour * 24 hours.

Additionally you could increment the date:

CopyyourDate.setDate(yourDate.getDate() + 1);

The neat thing about setDate is that if your date is out-of-range for the month, it will still correctly update the date (January 32 -> February 1).

See more documentation on setDate on MDN.

You have a problem with types. If days is 14, then it will yield 16th August correctly. However, if days is "14", then it will yield 2nd of March.

Solution:

  addDays(date: Date, days: number): Date {
    console.log('adding ' + days + ' days');
    console.log(date);
    date.setDate(date.getDate() + parseInt(days));
    console.log(date);
    return date;
  }

Just calculate on the Date object.

var today = new Date();

var startDate = new Date();
startDate.setDate(today.getDate() - 7);

var endDate = new Date();
endDate.setDate(today.getDate() + 7);

console.log('TODAY', today.toISOString().split('T')[0]);
console.log('START_DATE', startDate.toISOString().split('T')[0]);
console.log('END_DATE', endDate.toISOString().split('T')[0]);

Extending prototype in javascript may not be a good idea, especially in professional codebases.

What you want to do is extend the native Date class:

class MyCustomDate extends Date {

  addDays(days) {
    const date = new MyCustomDate(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
  }
  
}

const today = new MyCustomDate();

const nextWeek = today.addDays(7)

console.log(nextWeek)

This way, if one day Javascript implements a native addDays method, you won't break anything.