age and date calculation
Calculating age looks like just subtracting a birth date from today, but the result in years, months and days does not come from dividing by 365. It comes from testing, month by month, whether the birthday has already happened, and that difference in method explains life milestones, the zodiac, and the problem of anyone born on February 29.
The June 2026 birthday had already passed by the time the August reference date was measured, so the calculation did not need to borrow a year; only 2 months and 3 days remained on top of the 36 completed years.
1990 minus 1900 divided by 12 points to the Horse in the animal cycle; 1990 minus 1984 divided by 10 points to Yang Fire in the element cycle, and the two results combined give the year its full signature.
2026 is not divisible by 4, so the observed date falls one day earlier; 2028 is divisible by 4 (and is not a century year), so the birthday lands back on the exact date of birth.
It compares the birth date with a reference date and returns the exact civil age in years, months and days. The same engine also powers days lived, next birthday, age difference and future target age calculations.
It first assumes the plain difference between the years and tests whether, applying that difference, the birthday date would already have passed by the reference date. If it has not, it subtracts 1 from the year total and reruns the same test at the month level, "borrowing" time from the following year until the result matches the actual date.
Because the age in completed years only depends on whether the birthday has already passed, but the remaining months depend on exactly where the birth date and the reference date fall in the calendar. Two people born in different months of the same year, measured on the same day, would both be 36 completed years old, but with entirely different counts of extra months and days.
It is a second cycle, of 5 elements (wood, fire, earth, metal, water) combined with a yin or yang polarity, that runs underneath the 12-animal cycle and repeats every 10 years. Because 12 and 10 do not share the same period, the full combination of animal plus element plus polarity only repeats every 60 years, the least common multiple of the two cycles.
The exact date of birth only exists once every 4 years (with the Gregorian rule's century exception), but the age calculation does not get stuck because of that: in years without a February 29, the birthday occurrence is adjusted to February 28, the last valid day of that month that year, and it lands back exactly on the 29th as soon as the next leap year arrives.
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