Precision timekeeping with atomic clocks: evolution and future directions

Accurate timekeeping is foundational to modern society, supporting critical technologies such as global navigation satellite systems (GNSS), high-frequency financial trading, synchronization of communication networks, and the realization of SI base units. Here we review the evolution of timekeeping, with a particular emphasis on atomic clocks and their applications. We describe key dissemination methods, including Network Time Protocol (NTP), Precision Time Protocol (PTP), Global Navigation Satellite System (GNSS), and Two-Way Satellite Time and Frequency Transfer (TWSTFT). We further discuss emerging applications with stringent time-synchronization requirements, such as quantum communication and quantum computing. Finally, we discuss evolving trends in global timekeeping, including the redefinition of the SI second, lunar timekeeping, and the use of artificial intelligence and machine learning (AI/ML) for predictive control and resilience in timekeeping systems. This review outlines the evolution of timekeeping, focusing on atomic clocks and time dissemination methods, underpinning modern technologies such as GNSS and communication networks and enabling emerging applications such as quantum communication, while addressing future directions in global synchronization and SI second redefinition.