Cross Time calculation 
If you want to compute the same user's online time from server log, who comes from multiple clients, so you need to remove duplicate parts.
Installation
gem install cross_time_calculationUsage
require 'time'
require 'cross_time_calculation'
@t1 = DateTime.parse("20130512").to_time
@t2 = @t1 + 33
@ctc = CrossTimeCalculation.new
@ctc.add @t1, @t2
@ctc.total_seconds # => 33See more complicated example in unit test