A clock consists of two frames of reference. This is seen in an ordinary analogue clock, which is composed of two parts:
The space frame is at rest relative to the observer. The time frame is in uniform angular motion relative to the observer. Measurement of space and time requires both frames. The units marked on the space frame have dual significance: (a) as amounts of space or spatial angles, and (b) as amounts of time.
Each hand or pointer has its own time frame. The pointer signifies the present. Positions behind the pointer are the past. Positions in front of the pointer signify the future. This is shown by an annular distance clock:
If the roles of the space and time frames are interchanged, then what was the time frame becomes a space frame and will need multiple pointers or marks, whereas the space frame that becomes a time frame will need only one pointer or mark:
