isLeapYear method
是否是闰年
Implementation
bool isLeapYear() {
bool leapYear = false;
bool leap = ((this % 100 == 0) && (this % 400 != 0));
if (leap == true) {
leapYear = false;
} else if (this % 4 == 0) {
leapYear = true;
}
return leapYear;
}