NetSim is a discrete event simulator, and hence time doesn't increase in a continuous fashion as it would in a continous time simulator. The time progress is discrete and depends on the times at which events are to be processed.

Every device in NetSim has an instance of the Network Stack. The protocol engines are called based on the layer at which the protocols operate. For example, TCP is called during execution of Transport IN or Transport OUT events, while 802.11b WLAN is called during execution of MAC IN, MAC OUT, PHY IN and PHY OUT events.

When these protocols are in operation they in turn generate events for NetSim's discrete event engine to process. Each event gets added in the Simulation kernel by the protocol operating at the particular layer of the Network Stack. These events are then fetched by the Network Stack in order to execute the functionality of each protocol.  At the end of Simulation, Network Stack writes trace files and the Metrics files that assist the user in analyzing the performance metrics and statistical analysis.

Hence the best way to collect these results is to run simulations with the packet trace enabled and then calculate your metrics from the packet trace (with a comparison against the time stamp in the packet trace). Please see the section in the user manual relating to packet trace.