How to add weeks to date using javascript?

You can do this :

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

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.

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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

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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]);

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This should do what you're looking for.

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

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To parse the specific dd/mm/yyyy format and increment days with 14 , you can do something like split the parts, and create the date object with y/m/d given specfically. (incrementing the days right away) Providing the separator is always -, the following should work:

function LicenceToOccupy(acceptCompletionDate)
{
    var parts = acceptCompletionDate.split("/");
    var date1 = new Date(parts[2], (parts[1] - 1), parseInt(parts[0]) + 14); //month 0 based, day: parse to int and increment 14 (2 weeks)
    document.frmAccept.acceptLicence.value = date1.toLocaleDateString(); //if the d/m/y format is the local string, otherwise some cusom formatting needs to be done

}

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 date1.toLocaleDateString() 

Thiswill return you date1 as a String in the client convention

To create a new date date2 with 2 weeks more (2weeks = 27246060 seconds):

 var date2 = new Date(date1 + 60*60*24*7*2);

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