Care Plan Oversight

Care Plan Oversight codes cover a 30 day period and it is my understanding that billing for oversight should not happen until after the close of the month in which oversight is performed.
My question is: What should the date of service be? First day of the month for which billing is being submitted; last day of the month for which billing is being submitted; from-to date billing; or something else?
Thanks for your help.
Paula Lang

dates

Start date of CPO and stop date CPO for calendar month