How weeks are calculated
We first calculate the difference between two calendar dates as a whole number of days (using UTC calendar day ordinals so daylight saving transitions do not change the day count). The number of complete weeks is the integer quotient of days divided by seven; the remainder is the extra days.
completeWeeks = Math.floor(calendarDays / 7)extraDays = calendarDays % 7A real leap-year example
From January 1, 2000 to January 1, 2001, there are 366 calendar days because 2000 was a leap year. That is 52 complete weeks plus 2 days. By comparison, January 1, 2001 to January 1, 2002 contains 365 days: 52 weeks plus 1 day.
Age-years and weeks are different grids
The Years view uses a square from one birthday to the next. The Weeks view uses continuous seven-day blocks beginning on the birth date; a week may cross a birthday. The Days view shows the selected age-year as dated individual days, including February 29 when applicable.
Born on February 29?
The visualizer places February 29 birthdays on February 28 in years without that date. This is a chosen display rule for the calendar, not a universal legal birthday rule.
What does reference age mean?
The reference age simply defines the length of the visualized timeline. It does not calculate how long a person will live; people cannot infer their lifespan from this chart.