Ruby on Rails
HowToAddAMPMToDateHelper

Here is some sample code to change the select_hour function to include the AM/PM (meridian) indicator. Put this in lib/fix_date.rb.

module ActionView
  module Helpers
    module DateHelper

      def select_hour(datetime, options = {})
        hour_options = []

        0.upto(23) do |hour|
          hour_options << ((datetime && (datetime.kind_of?(Fixnum) ? datetime : datetime.hour) == hour) ?
          %(<option value="#{hour}" selected="selected">#{ampm(hour)}</option>\n) :
          %(<option value="#{hour}">#{ampm(hour)}</option>\n)
          )
        end

        select_html(options[:field_name] || 'hour', hour_options, options[:prefix], options[:include_blank], options[:discard_type])
      end

      def ampm(hour)
        case hour
          when 0     : "12 AM"
          when 1..11 : "#{hour} AM"
          when 12    : "12 PM"
          else         "#{hour-12} PM"
        end
      end
      
    end
  end
end

Then add this to the end of your config/environment.rb:

require 'lib/fix_date'

Since you have changed your environment, you will need to restart your web server before the change will take effect.

===

If you want the AM/PM as a separate drop-down, try this plugin:

http://code.google.com/p/rails-twelve-hour-time-plugin/