It is agreed that any loss of or damage to the Insured Property described in the schedule arising during any one period of 72 consecutive hours, caused by storm, tempest, flood and earthquake shall be deemed as a single event and therefore to constitute one occurrence with regard to the excesses provided for herein. For the purpose of the foregoing the commencement of any such 72 hour period shall be decided at the discretion of the Insured providing that there shall be no overlapping in any two or more 72 hour periods in the event of damage occurring over an extended period of time.