class DateTime

def seconds_until_end_of_day

DateTime.new(2012, 8, 29, 23, 59, 59).seconds_until_end_of_day # => 0
DateTime.new(2012, 8, 29, 12, 34, 56).seconds_until_end_of_day # => 41103
DateTime.new(2012, 8, 29, 0, 0, 0).seconds_until_end_of_day # => 86399

Returns the number of seconds until 23:59:59. 
def seconds_until_end_of_day
  end_of_day.to_i - to_i
end