session lenght

Hi every one first of all i want to thank you all for your help for my every question

i want to ask that i have executed the code of packet sniffer which using pcap.h.i want to find the session length which i think can be calculated by time when last packet was capture minus time when first packet was capture am i right ?
if i am right how do i save the time stamp info of only first and last packet as i is continuously overwritten when new packets are arriving.and how to save packet info of different session like i am running yahoo as well as hotmail i want to store the packet info of these 2 different session in a different file how can i do this?
thanks thanks alot.