When Is a Year a Leap Year?

Triggered by the preceding post, an afterthought:

Do you know how to tell whether a year is a leap year? Or do you only *think* you know?

Here's how it goes: A year is a leap year if

• It is evenly divisible by 4
  • but not evenly divisible by 100
    • unless it is evenly divisible by 400
      • and not evenly divisible by 4000 (Gotcha!)