Years between dates
A complete year runs from a date to the same date the following year. From 1 January 2000 to 31 December 2026 is 26 complete years, because the twenty-seventh has not finished. The anniversary falls the next day.
How to count years between dates
The completed-year convention is why an age ticks over on a birthday rather than at any point during the year. It is also why 29 February birthdays are a genuine legal question: in England a leapling legally turns 18 on 1 March, while in some other jurisdictions it is 28 February. The arithmetic here follows the common convention of the anniversary date, clamped to 28 February in a common year.
Questions
Count complete anniversaries. The final year counts only once its anniversary has passed.
Because years are counted from anniversary to anniversary, not by calendar year number.
It clamps to 28 February in common years here. The legal answer varies by jurisdiction.
Divide the days by 365.2425, the average Gregorian year.
No; 366 in a leap year, which is roughly one year in four.