JMeter results' timestamp field refers to the time URL was requested. If you want to know how much time a thread spent you should sum elapsed time field.
The end time of a thread is the first timestamp field plus the sum of elapsed time values for this thread's requests. If you group up requests inside a Transaction Controller you could have these results calculated by JMeter nicely. Try also the Response Times Percentiles listener, from JMeter-Plugins' package, with the Transaction Controller approach. You should see the variation of total elapsed times between theads. If you intend to measure how much requests your app can handle in a period of time then I suggest you to try Throughput Shaping Timer, from JMeter-Plugins' package.
