How do I get a date in YYYY-MM-DD format?

The below code is a way of doing it. If you have a date, pass it to the convertDate() function and it will return a string in the YYYY-MM-DD format:

var todaysDate = new Date();

function convertDate(date) {
  var yyyy = date.getFullYear().toString();
  var mm = (date.getMonth()+1).toString();
  var dd  = date.getDate().toString();

  var mmChars = mm.split('');
  var ddChars = dd.split('');

  return yyyy + '-' + (mmChars[1]?mm:"0"+mmChars[0]) + '-' + (ddChars[1]?dd:"0"+ddChars[0]);
}

console.log(convertDate(todaysDate)); // Returns: 2015-08-25

By using Moment.js library, you can do:

var datetime = new Date("2015-09-17 15:00:00");
datetime = moment(datetime).format("YYYY-MM-DD");

Yet another way:

var today = new Date().getFullYear()+'-'+("0"+(new Date().getMonth()+1)).slice(-2)+'-'+("0"+new Date().getDate()).slice(-2)
document.getElementById("today").innerHTML = today

<div id="today">

Just use the built-in .toISOString() method like so: toISOString().split('T')[0]. Simple, clean and all in a single line.

var date = (new Date()).toISOString().split('T')[0];
document.getElementById('date').innerHTML = date;

<div id="date"></div>

Please note that the timezone of the formatted string is UTC rather than local time.