start
if the MPINavigatable.MPILocation is currently not available, start represents the start date of the MPINavigatable.MPILocation; otherwise, nil.
Example: "2019-09-01T04:00:00.000Z"
if the MPINavigatable.MPILocation is currently not available, start represents the start date of the MPINavigatable.MPILocation; otherwise, nil.
Example: "2019-09-01T04:00:00.000Z"