Hmm, I just tested it in irb. Looks like when you subtract two objects from the Time class, the difference is a float.

1.9.3p429 :007 > start_time = Time.new(2000, 1, 1, 0, 0, 1) ; end_time = Time.new(2000, 1, 1, 0, 0, 2) ; (start_time - end_time).class
=> Float