Time select elements in rails use 0-23 for the hour. I'm used to 1-12 AM/PM.

date_time_selector_with_meridian.rb

#
# <%= form.select_time :start, :meridian => true %>
#
module ActionView
  module Helpers
    class DateTimeSelector
      def select_time_with_meridian
        html = select_time_without_meridian
        if @options[:meridian]
          id = input_id_from_type(:hour)
          onchange = <<-EOS
            if (this.value == 'AM') {
              $A($('#{id}').options).each(function(o, i) {
                o.value = i + 1;
                if (o.value == 12) o.value = 0;
                if (o.value < 10) o.value = '0' + o.value;
              });
            } else {
              $A($('#{id}').options).each(function(o, i) {
                o.value = i + 13;
                if (o.value == 24) o.value = 12;
              });
            }
            EOS
          value = hour < 12 ? 'AM' : 'PM'
          html += "<select onchange=\"#{onchange}\"><option#{' selected="selected"' if value == 'AM'}>AM</option><option#{' selected="selected"' if value == 'PM'}>PM</option></select>"
        end
        html
      end
      alias_method_chain :select_time, :meridian

      def select_hour_with_meridian
        if @options[:meridian]
          if @options[:use_hidden] || @options[:discard_hour]
            build_hidden(:hour, hour)
          else
            h = hour % 12
            h = 12  if h == 0
            html = build_options_and_select(:hour, h, :start => 1, :end => 12)
            if hour < 12
              html.gsub('value="12"', 'value="00"')
            else
              html.gsub(/value="(\d+)"/) do |match|
                value = $1.to_i + 12
                value = 12  if value == 24
                "value=\"#{value}\""
              end
            end
          end
        else
          select_hour_without_meridian
        end
      end
      alias_method_chain :select_hour, :meridian
    end
  end
end