The cycles of the earth refers to the rotation of the planet around the sun. These rotations are divided into units of "time" that vary between one another based upon the distinction of length. Sun rise and sunset are used to denote a length of time in a day. The total number of days it takes for the earth to complete one rotation around the sun is defined as a year. For the sake of utility, we arbitrarily divided each year into months.
Thus "time" is divided multiple ways across the spectrum of utility: hours as a unit of time to define sunrise and sunset or "one day"; the number of days in a given month; 12 months in a year.