A grandmaster clock is the primary time source in a Precision Time Protocol (PTP) domain. Other devices on the network synchronize to it, either directly or through intermediate clocks, so they share a common time base for logs, control actions, and event ordering.
In cyber security, the grandmaster matters because time is part of system integrity. If attackers delay packets, create path asymmetry, spoof GPS timing, or tamper with the clock source, devices can drift from the real reference while still appearing normal. That can corrupt audit logs, disturb protection logic, and make incident reconstruction unreliable. Defenders therefore protect grandmasters with redundancy, authenticated timing where available, holdover capability, and monitoring for offset, delay, and drift anomalies. In industrial and power systems, the grandmaster is not just a convenience; it is a trusted control dependency.