How to add weeks to date using javascript?

You're assigning date1 to be a Date object which represents the string you pass it. What you're seeing in the acceptLicense value is the toString() representation of the date object (try alert(date1.toString()) to see this).

To output as you want, you'll have to use string concatenation and the various Date methods.

var formattedDate = date1.getDate() + '/' + (date1.getMonth() + 1) + '/' + date1.getFullYear();

In terms of adding 2 weeks, you should add 14 days to the current date;

date1.setDate(date.getDate() + 14);

... this will automatically handle the month increase etc.

In the end, you'll end up with;

var date1 = new Date(acceptCompletionDate);
date1.setDate(date1.getDate() + 14);
document.frmAccept.acceptLicence.value = date1.getDate() + '/' + (date1.getMonth() + 1) + '/' + date1.getFullYear();

N.B Months in JavaScript are 0-indexed (Jan = 0, Dec = 11), hence the +1 on the month.

Edit: To address your comment, you should construct date as follows instead, as the Date argument is supposed to be "A string representing an RFC2822 or ISO 8601 date." (see here).

var segments = acceptCompletionDate.split("/");
var date1 = new Date(segments[2], segments[1], segments[0]);

You can do this :

const numWeeks = 2;
const now = new Date();
now.setDate(now.getDate() + numWeeks * 7);

or as a function

const addWeeksToDate = (dateObj,numberOfWeeks) => {
  dateObj.setDate(dateObj.getDate()+ numberOfWeeks * 7);
  return dateObj;
}

const numberOfWeeks = 2 
console.log(addWeeksToDate(new Date(), 2).toISOString());

You can see the fiddle here.

According to the documentation in MDN

The setDate() method sets the day of the Date object relative to the beginning of the currently set month.

This might not answer the question per se, but one can find a solution with these formulas.

6.048e+8 = 1 week in milliseconds

Date.now() = Now in milliseconds

Date.now() + 6.048e+8 = 1 week from today

Date.now() + (6.048e+8 * 2) = 2 weeks from today

new Date( Date.now() + (6.048e+8 * 2) ) = Date Object for 2 weeks from today