l see, in-network synchronization timescales at the nanosec- ond (one billionth of a second) are often quoted. While some applications in a cellular network do need to know the actual, true, time, for example, call logging for an itemized bill and indeed some user applications themselves, such as a clock/calendar on a handset, in many cases, it is relative time that matters, as in what is the difference in time between two events. Additionally, at least in O-RAN applica- tions, such events occur at a timescale well below a second-for example, the alignment of transmitted data frames for carrier aggregation. In these instances, it is more common to use the term "phase," to mean the time within a given second. For example, it will normally be enough to know that one event occurred 20% of the way through a second, whereas another occurred 55% of the way through the same second, without even actually knowing which second we're talking about. Even though time and phase have distinct, different, meanings, for the remainder of this chapter, we will use the term time in all cases, even if what we actually care about is the phase, since the distinction between the two becomes irrelevant for most discussions. What is time? Time and frequency are clearly very closely related. We have already seen that frequency defined in terms of cycles per seconds, and when we talk about time, what we really mean is the number of seconds, or parts of seconds, since something. The big difference though is that something-the starting point from which we count seconds. Formally, this was referred to as the epoch and its selection is rather arbitrary. Most of the Western World uses an epoch somewhat loosely based on events described in the Christian bible, with time measured in seconds, minutes, hours, days, months, and year from a start of 1 BCE, or more correctly, CE 1 (for Common Era). However, elsewhere in the world, different starting points are used. For example, the Hebrew calendar defines "year 1" as approximately 3760 years before th