“Kalachakra” means cycles of time, with time being a measurement of change, both externally in the world and the universe, and internally within the body.
As such, it can be measured in many ways. Externally, there are the cycles of the orbits of the planets, the months and seasons of the year, the phases of the moon, the hours of a day and so on, not to mention historical cycles and periods of peace and war. Internally, there are the cycles of the periods of life (infancy, childhood, youth, adulthood, and old age), the menstrual, sleep, and circadian cycles, the number of breaths in a day, and the like.
These external and internal cycles closely parallel each other. Just as stars, galaxies and universes go through cycles of formation, endurance, dissipation and extinction, humans undergo the cycles of birth, a lifetime, old age and death together with a bardo, or in-between period. Further, both the internal and external cycles have been repeating beginninglessly with the continuing rebirth of universes and lifetimes.