gistfile1.js

$(document).ready(function() {
	$('body').on('click', '.set-to-now', function(e) {
		e.preventDefault();
		var target_selector = $(this).attr('data-target');
		var year    = $(target_selector+'_1i');
		var month   = $(target_selector+'_2i');
		var day     = $(target_selector+'_3i');
		var hour    = $(target_selector+'_4i');
		var minute = $(target_selector+'_5i');
		// var current = [year, month, day, hour, minute].map(function(item) {
		// 	return item.val();
		// });

		var now = new Date();
		year.val(   now.getFullYear() );
		month.val(  now.getMonth()+1 );
		day.val(    now.getDate() );
		hour.val(   now.getHours() );
		minute.val( now.getMinutes() );		
	});
});

Assuming your model is "Post" and the form has something like:

<%= f.datetime_select :published_at %> <a href="#" class="set-to-now" data-target="#post_published_at">NOW</a>