HTML5 Input datetime-local default value of today and current time

You can make it shorter:

<input type="datetime-local" id="cal">

window.addEventListener('load', () => {
  var now = new Date();
  now.setMinutes(now.getMinutes() - now.getTimezoneOffset());

  /* remove second/millisecond if needed - credit ref. https://stackoverflow.com/questions/24468518/html5-input-datetime-local-default-value-of-today-and-current-time#comment112871765_60884408 */
  now.setMilliseconds(null)
  now.setSeconds(null)

  document.getElementById('cal').value = now.toISOString().slice(0, -1);
});

It's possible. By using a JQuery function, you can have a really complete solution.

Here is an example.

JSFiddle http://jsfiddle.net/v8MNx/1/

HTML

<form action="demo.html" id="myForm">
    <p>
        <label>Date:</label>
        <input type="datetime" name="anniversaire" id="anniversaire"/>
    </p>
    <input type="submit" value="Submit"/>
</form>

JQuery:

//Function found here: https://gist.github.com/ryanburnette/8803238
$.fn.setNow = function (onlyBlank) {
  var now = new Date($.now())
    , year
    , month
    , date
    , hours
    , minutes
    , seconds
    , formattedDateTime
    ;

  year = now.getFullYear();
  month = now.getMonth().toString().length === 1 ? '0' + (now.getMonth() + 1).toString() : now.getMonth() + 1;
  date = now.getDate().toString().length === 1 ? '0' + (now.getDate()).toString() : now.getDate();
  hours = now.getHours().toString().length === 1 ? '0' + now.getHours().toString() : now.getHours();
  minutes = now.getMinutes().toString().length === 1 ? '0' + now.getMinutes().toString() : now.getMinutes();
  seconds = now.getSeconds().toString().length === 1 ? '0' + now.getSeconds().toString() : now.getSeconds();

  formattedDateTime = year + '-' + month + '-' + date + 'T' + hours + ':' + minutes + ':' + seconds;

  if ( onlyBlank === true && $(this).val() ) {
    return this;
  }

  $(this).val(formattedDateTime);

  return this;
}

$(function () {
    // Handler for .ready() called.
    $('input[type="datetime"]').setNow();

});