How age is counted
Age = whole years from birth → target, then remaining months, then days
We count complete years first, then the leftover whole months and days — borrowing days from the previous month when the day-of-month hasn't been reached yet. This matches how a birthday is reckoned, and correctly handles leap years and varying month lengths.
How your age is calculated
Age is counted the way people naturally think about it: complete years first, then the leftover whole months and days. When the day-of-month hasn't been reached yet, we borrow days from the previous month — exactly how a birthday is reckoned.
Change the "age at" date to find your age on any past or future day, and see the same span expressed as total months, weeks, days, and hours.