plusDays method
Returns date in future given number of days
Implementation
HDate plusDays(int numDays) {
if (numDays == 0) return this;
if (numDays < 0) return minusDays(-numDays);
int year = this.year;
int month = this.month;
int day = this.day;
for (; numDays > 0; --numDays) {
day++;
if (day > daysInMonth(year, month)) {
day = 1;
month++;
if (month > 12) {
month = 1;
year++;
}
}
}
return HDate(year, month, day);
}