A jQuery method that sets datetime fields to the current UTC time.

setnow.jquery.js

$.fn.setNow = function (onlyBlank) {
  var now = new Date($.now());
  
  var year = now.getFullYear();
  
  var month = (now.getMonth() + 1).toString().length === 1 ? '0' + (now.getMonth() + 1).toString() : now.getMonth() + 1;
  var date = now.getDate().toString().length === 1 ? '0'         + (now.getDate()).toString()      : now.getDate();
  var hours = now.getHours().toString().length === 1 ? '0'       + now.getHours().toString()       : now.getHours();
  var minutes = now.getMinutes().toString().length === 1 ? '0'   + now.getMinutes().toString()     : now.getMinutes();
  var seconds = now.getSeconds().toString().length === 1 ? '0'   + now.getSeconds().toString()     : now.getSeconds();
  
  var formattedDateTime = year + '-' + month + '-' + date + 'T' + hours + ':' + minutes + ':' + seconds;

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

correction:

month = (now.getMonth() + 1).toString().length === 1 ? '0' + (now.getMonth() + 1).toString() : now.getMonth() + 1;

This is probably a better way to do it:

$('input[type=datetime-local]').prop('valueAsNumber', Math.floor(new Date() / 60000) * 60000); // 60seconds * 1000milliseconds

It might not work for all browsers. You could feature detect.

If you do:

var x = $('input[type=datetime-local]')[0];

Into your console for each browser you can auto-complete to see what they have.

I've discovered with this there's some support for milliseconds as well which is not included here.

Also you should just make it take a parameter. All now is is new Date(). It makes more sense to have that as a parameter so it's more flexible. You can call it setDate instead or something.

Date type coerces to a long of milliseconds unixtime (always UTC I believe) or something similar with numeric operations.

You can also do a getter with:

new Date($(this).prop('valueAsNumber'));

I made the change to month suggested by @eayavuz.