constexpr bool is_leap() const noexcept;
*this represents a leap year in the proleptic Gregorian calendar.
*this represents a leap year if the stored year value
- is divisible by 4 and not divisible by 100; or
- is divisible by 400.
 Return value
*this represents a leap year, otherwise false.
2020 is a leap year 2021 is not a leap year 2000 is a leap year 3000 is not a leap year