The time scale in a system supporting IEEE 1588 is defined by the time scale of the grandmaster clock. The grandmaster clock is the clock at the root of the parent child hierarchy defined by IEEE 1588 Boundary Clocks and the operation of the IEEE 1588 protocol. In a system with no boundary clocks one of the ordinary clocks in the subnet will be selected by the protocol as the master (and by default the grandmaster) clock of the system.
The time scale established by the grandmaster clock depends on the design of the grandmaster clock. Typically a clock will consist of a crystal oscillator driving a counter. The frequency of the oscillator defines a 'second' in the 1588 world and how the 1588 'second' compares to the internationally defined second, the SI second. The resolution of the counter establishes the resolution of the time scale. The drift characteristics of the crystal define the stability of the time scale.
The initialization of the counter defines the 'epoch' or origin of the time scale. Implementations typically will provide a mechanism for 'setting' the time in the grandmaster clock. IEEE 1588 specifies 'management messages' that may be used for this purpose. If only 'relative time' is required, then there is no need to initialize the time scale of the grandmaster clock. Implementations may also provide a battery backed up clock to maintain the time scale over power outages.
If the time scale of the 1588 grandmaster clock is UTC then the time scales of the rest of the 1588 clocks in the system will also be UTC. The grandmaster clock may be 'set' to UTC time using the management messages. In this case the time throughout the 1588 system will track UTC with an accuracy depending on the precision of the initialization and the drift of the oscillator of the grandmaster clock. With inexpensive crystal oscillators this drift will cause departures from UTC of perhaps a fraction of a second per day although the relative synchronization between the 1588 clocks remains very precise.
If close agreement with UTC is to be maintained then the grandmaster clock itself must be synchronized to a high quality source of UTC time. For example, this may be done by synchronizing the grandmaster clock to the GPS (Global Positioning Satellite) system maintained by the US Department of Defense. Other alternatives are to synchronize the grandmaster clock to an external clock participating in NTP (Network Time Protocol) or to an atomic clock. Of these the GPS system will usually be the most practical for high accuracy systems. Prototype implementations of 1588 systems in which the grandmaster is synchronized to a GPS receive have demonstrated time scales that track UTC within the nominal accuracy of the GPS system.