In this talk I present the clock synchronization problem – many times. This research area’s natural habitat is distributed computing, as by definition synchronizing clocks implies a distributed system. However, as I will seek to convince you in this talk, it has become a key issue in the development of reliable, scalable hardware systems as well. I will discuss a wide range of theoretical and practical issues arising from the challenge of providing a precise time base in face of various sources of uncertainty and faults, and point at a number of open problems that I consider of interest to the distributed computing community.