" There is no difficulty in synchronizing two clocks in the same reference frame: one procedure is to send a light pulse simultaneously to two clocks from a point midway between them, with the two operators setting their clocks to a predetermined time when the pulses arrive. In a thought experiment it is often helpful to imagine a large number of synchronized clocks distributed conveniently in a single frame of reference. Only when a clock is moving relative to a given reference frame do ambiguities of synchronization arise." University Physics Sears Zemansky Young 1980 page 254

In a thought experiment, place four cesium clocks at the four points of an equilateral tetrahedron.
Let the length of each side be equal to the distance that light travels in one- hundredth of a second (1860 miles) and be permanently physically attached to the center of this tetrahedron and to each other.
Send 4 laser signals, from the tetrahedron center to all four locations at the same moment.
As each location receives its signal, it records the moment and initializes the clock at its location.
Physically, each of these four points is of equal distance from the tetrahedron center, and due to the four light signals travelling at the same speed, it allows these four points to establish a common time frame.
These four points, along with their common time frame can be extended to include an entire inter-coordinated system that could encompasses the entire universe.
The critical factor is that these four points never change location with respect to each other.