The timestamp is the number of milliseconds from the previous message. The first message will always be 0, the second message will have the time from the first message, the third message shows the time from the second message, and so on.
In order to broadcast the session key in the field, a provisioning key must be set in advance. The provisioning key encrypts the session key and ensures that no unencrypted key is ever broadcast. At the factory or at some point prior to field ...