Evolution of Timing Services From 5G-A Toward 6G

5G-Advanced and 6G networks will serve as critical infrastructure for society and will enable a new generation of immersive use cases, such as the metaverse, wherever people roam. Absolute time is an essential component to ensure critical use cases, synchronize media playout, and timestamp events to be used in machine learned contexts. Until now, timing on the go has mainly been acquired by satellite, e.g., Global Navigation Satellite System (GNSS), but a terrestrial timing solution using cellular networks is required to extend coverage to deep indoors and to offer resilient operation in GNSS-denied environments (e.g., due to GNSS signal interference, jamming and spoofing). In this paper, we detail the use cases for timing to be provisioned by 5G-Advanced and 6G networks. Then, we discuss the architectural enablers for timing as a service in current 5G and 5G-Advanced standard and timing resiliency enablers under discussion in the 3rd Generation Partnership Project (3GPP). Finally, we discuss the gaps and research challenges to be solved in 6G for future-proof timing solutions.