module Sinatra::Helpers

def last_modified(time)

with a '304 Not Modified' response.
equal or later than the time specified, execution is immediately halted
When the current request includes an 'If-Modified-Since' header that is

DateTime, or other object that responds to +to_time+.
and halt if conditional GET matches. The +time+ argument is a Time,
Set the last modified time of the resource (HTTP 'Last-Modified' header) 
def last_modified(time)
  return unless time
  time = time_for time
  response['Last-Modified'] = time.httpdate
  return if env['HTTP_IF_NONE_MATCH']
  if (status == 200) && env['HTTP_IF_MODIFIED_SINCE']
    # compare based on seconds since epoch
    since = Time.httpdate(env['HTTP_IF_MODIFIED_SINCE']).to_i
    halt 304 if since >= time.to_i
  end
  if (success? || (status == 412)) && env['HTTP_IF_UNMODIFIED_SINCE']
    # compare based on seconds since epoch
    since = Time.httpdate(env['HTTP_IF_UNMODIFIED_SINCE']).to_i
    halt 412 if since < time.to_i
  end
rescue ArgumentError
end