Adding Halfhour Traffic

Could someone help me on how to add half-hour erlang traffic so that you determine the busy hour. Our switch is able to retrieve only offered traffic (in erlangs) for 30 minutes, as well as the total number of attempts in that period.We want to find the busy hour traffic by somehow combining the two halves.How should we proceeed?Should we directly add the offered erlangs or should we use the total number of attempts and the switch GOS to come up with the BHT ?