_form.html.erb

#
# This is using gem called simple_form by Jose Valim, but it can work with normal Rails calendar_select form inputs
#

<form....>

  <%= f.input :target_date, :input_html => {:rel => '{"changeYear":true, "changeMonth":true, "minDate":"+1D", "maxDate":"+12M"}'} %>

</form>

application.js

/**
 * Shamlessly copied from http://snipt.net/boriscy/datetime-jquery-formtastic/
 *
 * This did not function with formtastic version 0.9.10
 * I had to move the live("change") event below the initializing part
 * I also had to adjust the jQuery selections as formtastic uses <ol>'s to
 * seperate every select box.
 */
$(document).ready(function() {

    // Define the dateFormat for the datepicker
    $.datepicker._defaults.dateFormat = 'dd M yy';
    
    /**
     * Replaces the date or datetime field with jquey-ui datepicker
     */
    $('.date, .datetime').each(function(i, el) {
        var input = document.createElement('input');
        $(input).attr({'type': 'text', 'class': 'ui-date-text'});
        // Insert the input:text before the first select
        $(el).find("select:first").before(input);
        $(el).find("select:lt(3)").hide();
        // Set the input with the value of the selects
        var values = [];
        $(el).find("select:lt(3)").each(function(i, el) {
            var val = $(el).val();
            if(val != '')
                values.push(val);
        });
        if( values.length > 1) {
            d = new Date(values[0], parseInt(values[1]) - 1, values[2]);
            $(input).val( $.datepicker.formatDate($.datepicker._defaults.dateFormat, d) );
        }

        $(input).datepicker( jQuery.parseJSON($(el).find("select:first").attr('rel')) );
    });

    /**
     * Sets the date for each select with the date selected with datepicker
     */
    $('input.ui-date-text').live("change", function() {
        var sels = $(this).closest('.date, .datetime').find("select:lt(3)");
        var d = $.datepicker.parseDate($.datepicker._defaults.dateFormat, $(this).val() );
        
        $(sels[0]).val(d.getFullYear());
        $(sels[1]).val(d.getMonth() + 1);
        $(sels[2]).val(d.getDate());
    });

});