plusYears method
Adds the specified number of years.
Handles leap year edge cases properly.
Example:
final leap = ZonedDateTime.parse('2020-02-29T12:00:00Z'); // Leap year
final next = leap.plusYears(1); // 2021-02-28T12:00:00Z (adjusted)
Implementation
ZonedDateTime plusYears(int years) {
final newLocalDateTime = _localDateTime.plusYears(years);
return ZonedDateTime.of(newLocalDateTime, _zone);
}