var date = new Date(); // Now
date.setDate(date.getDate() + 30); // Set now + 30 days as the new date
console.log(date);

Updated answer (2018)

One way to add 30 days to a date string is to parse it to a Date, add 30 days, then format it back to a string.

Date strings should be parsed manually, either with a bespoke function or a library. Either way, you need to know the format to know if it's been parsed correctly, e.g.

// Given a string in m/d/y format, return a Date
function parseMDY(s) {
  var b = s.split(/\D/);
  return new Date(b[2], b[0]-1, b[1]);
}

// Given a Date, return a string in m/d/y format
function formatMDY(d) {
  function z(n){return (n<10?'0':'')+n}
  if (isNaN(+d)) return d.toString();
  return z(d.getMonth()+1) + '/' + z(d.getDate()) + '/' + d.getFullYear();
}

// Given a string in m/d/y format, return a string in the same format with n days added
function addDays(s, days) {
  var d = parseMDY(s);
  d.setDate(d.getDate() + Number(days));
  return formatMDY(d);
}

[['6/30/2018', 30],
 ['1/30/2018', 30], // Goes from 30 Jan to 1 Mar
 ['12/31/2019', 30]].forEach(a => {
  console.log(`{addDays(...a)}`);
});

If the "30 days" criterion is interpreted as adding a month, that is a bit trickier. Adding 1 month to 31 January will give 31 February, which resolves to 2 or 3 March depending on whether February for that year has 28 or 29 days. One algorithm to resolve that is to see if the month has gone too far and set the date to the last day of the previous month, so 2018-01-31 plus one month gives 2018-02-28.

The same algorithm works for subtracting months, e.g.

/**
 * @param {Date} date - date to add months to
 * @param {number} months - months to add
 * @returns {Date}
*/
function addMonths(date, months) {

  // Deal with invalid Date
  if (isNaN(+date)) return;
  
  months = parseInt(months);

  // Deal with months not being a number
  if (isNaN(months)) return;

  // Store date's current month
  var m = date.getMonth();
  date.setMonth(date.getMonth() + months);
  
  // Check new month, if rolled over an extra month, 
  // go back to last day of previous month
  if (date.getMonth() != (m + 12 + months)%12) {
    date.setDate(0);
  }
  
  // date is modified in place, but return for convenience
  return date;
}

// Helper to format the date as D-MMM-YYYY
// using browser default language
function formatDMMMY(date) {
  var month = date.toLocaleString(undefined,{month:'short'});
  return date.getDate() + '-' + month + '-' + date.getFullYear();
}

// Some tests
[[new Date(2018,0,31),  1],
 [new Date(2017,11,31), 2],
 [new Date(2018,2,31), -1],
 [new Date(2018,6,31), -1],
 [new Date(2018,6,31), -17]].forEach(a => {
   let f = formatDMMMY;
   console.log(`${f(a[0])} plus ${a[1]} months: ${f(addMonths(...a))}`); 
});

Of course a library can help with the above, the algorithms are the same.

Original answer (very much out of date now)

Simply add 30 days to todays date:

var now = new Date();
now.setDate(now.getDate() + 30);

However, is that what you really want to do? Or do you want to get today plus one month?

You can convert a d/m/y date to a date object using:

var dString = '9/5/2011';
var dParts = dString.split('/');
var in30Days = new Date(dParts[2] + '/' +
                        dParts[1] + '/' +
                        (+dParts[0] + 30)
               );

For US date format, swap parts 0 and 1:

var in30Days = new Date(dParts[2] + '/' +
                        dParts[0] + '/' +
                        (+dParts[1] + 30)
               );

But it is better to get the date into an ISO8601 format before giving it to the function, you really shouldn't be mixing date parsing and arithmetic in the same function. A comprehensive date parsing function is complex (not excessively but they are tediously long and need lots of testing), arithmetic is quite simple once you have a date object.

var date = new Date();
date.setDate(date.getDate() + 30); // add 30 days 
console.log(date);

You can create one using Date.prototype.setDate():

Date.prototype.addDays = function(days) {
    var date = new Date(this.valueOf());
    date.setDate(date.getDate() + days);
    return date;
}

var date = new Date();

console.log(date.addDays(5));

This takes care of automatically incrementing the month if necessary, as noted here. For example:

8/31 + 1 day will become 9/1.

The problem with using setDate directly is that it's a mutator and that sort of thing is best avoided. ECMA saw fit to treat Date as a mutable class rather than an immutable structure.

For leading zeros:

// add leading zero if the length equals 1
if (month < 10) month = "0" + month;
if (day < 10)   day   = "0" + day;

Be sure to add 1 to your month prior to using this code, too, since getMonth() returns a 0 for January, and so on:

 var month = d.getMonth() + 1;

You can follow this approach

• parse the input date value to date using split and date constructor
• Add days using setDate
• Set the new date value to output date.

Demo

document.querySelector("#addDays").addEventListener("click", function() {
  var invoiceDate = document.querySelector("#invoiceDate").value;
  var days = Number(document.querySelector("#days").value);
  var dueDateElement = document.querySelector("#dueDate");

  if (!isNaN(days) && invoiceDate.length) {
    invoiceDate = invoiceDate.split("-");
    invoiceDate = new Date(invoiceDate[0], invoiceDate[1] - 1, invoiceDate[2]);
    invoiceDate.setDate(invoiceDate.getDate() + days);
    dueDateElement.valueAsDate = null;
    dueDateElement.valueAsDate = invoiceDate;
    //console.log(invoiceDate, dueDateElement.value);
  }
});

Invoice Date <input type="date" id="invoiceDate"> <br><br> Add Days <input type="text" id="days"> <br><br>

<button id="addDays">Add Days</button> <br><br> Due Date <input type="date" id="dueDate">

setDate() doesn't return a Date object, it returns the number of milliseconds since 1 January 1970 00:00:00 UTC. You need separate calls:

var date = new Date();
date.setDate(date.getDate() - 30);
var dateString = date.toISOString().split('T')[0]; // "2016-06-08"

What you're seeing is just the default representation of a date - dates are stored as number of milliseconds since datum.

If you format the value as an ISO string (for example) you'll see it actually has the value you expected

let storePlanStartDate = new Date("2021-01-02T15:12:05.000Z");

storePlanStartDate.setDate(storePlanStartDate.getDate() + 30)

console.log(storePlanStartDate.toISOString());