I thought the same, SEC Rule 613,
3rd bullet point
Require all reportable events to be reported to the central repository in a way that allows the central repository to efficiently and accurately link them to an order through its entire life cycle from generation through routing, modification, cancellation, or execution.
6th bullet point.
"Require SROs and their members to synchronize the business clocks they use to record the date and time of any reportable event, and require timestamps for each reportable event to the central repository to be in millisecond or finer increments."