How date difference works
The end date minus the start date gives the total elapsed days. On top of that, calendar years and months are counted: full years are extracted first, then remaining full months, with leftover days at the end. If the day-of-month of the end date is earlier than the start, one month is borrowed and its days are added back — the same approach a calendar app uses.
Worked example. From 2000-01-01 to 2020-01-01 is exactly 20 years, 0 months, 0 days — and 7,305 total days (including 5 leap-year days in 2000, 2004, 2008, 2012, and 2016).
Finding your age
Set the start date to your date of birth and leave the end date as today. The Years card shows your current age, and Total days shows how many days you have been alive.
Leap years
The calculator handles leap years automatically. A year is a leap year if it is divisible by 4, except for century years which must be divisible by 400.