The Personal Year cycles in particular are popular, because not only are they strongly felt, they are also easy to calculate. (See Calculation Methods.)
Personal Year cycles
Personal Year cycles are influenced by Universal Year cycles, which is reflected in the way the Personal Year cycle is calculated: Add your Month of Birth, your Day of Birth, and the current (universal) year together, and then reduce to a single digit. In other words, add your Month and Day of Birth to the Universal Year cycle. A fusion, so to speak, of your Birth Date and the cycles of the calendar year. It is because of this fusion, that the Personal Year cycles run concurrent with the calendar year cycles.
But, if that is true, why do some numerologists feel that the Personal Year cycles start at your birth day and not at the beginning of the year?
The answer is in the overlapping cusp periods.
Think of your birth day as a powerful force that pulls and guides all your cycles, including the Personal Year cycle. The force your month and day of birth bestows upon your Personal Year cycle pulls this cycle towards maturity. Just like the moon pulls the tide, your day of birth pulls your Personal Year cycle. For example, a person born in February, has a short cusp period of perhaps one or two months at the beginning of the Personal Year cycle, reaching maturity sometime in the late part of January, which is followed by a five or six months period from January through July or August during which the Personal Year cycle is fully mature, after which the cycle slowly decreases over the next five or six months. During the last couple of months of this period of decrease, the new cycle is already forcing its way in. See the graphic below.
The left column shows the month of birth.
Red shows the period when your current Personal Year cycle is in full force
Yellow shows when the old cycle is still influencing you, but is on its way out.
Blue/purple highlights the time when the next cycle starts to make its presence known.
The list of months on the left reflects the months of birth.
Personal Month Cycles
Personal Month cycles are calculated by adding the current month to your Personal Year cycle. And, just like Personal Year cycles, there is a cusp period on each end. This time, however, the cusp period is not controlled by your month of birth, but by your day of birth - in a similar fashion but on a smaller scale, as the yearly cycles.
The fact that Personal Month cycles are based on Personal Year cycles, be it January or December, is another indication the Personal Year runs from January through December. (No numerologist would start the count of your Personal Month cycles after the month of your birth.)
Numerology and Personal Day cycles.
Personal Day cycles also have cusp periods. In Personal Day cycles they are based on the time of your birth. People born in the evening, or early morning, sometimes feel the Personal Day cycles are more accurate if shifted one day forward or backward, respectively.