년도, 월, 일 을 가지고 요일을 계산한다.
/**
Zeller's congruence
@param day the day of the month
@param month the month
@oaran year the year
return the weekday (0 = Monday, 1 = Tuesday, ..., 6 = Sunday)
*/
int zeller(int day, int month, int year)
{
int modifiedMonth[] = { 0, // not used
13, 14, 3, 4, // month : 1 ~ 4
5, 6, 7, 8, // month : 5 ~ 8
9, 10, 11, 12 }; // month : 9 ~ 12
int w;
int mm = modifiedMonth[month];
w = ( day + 5
+ ( 26 * (mm + 1) ) / 10
+ ( 5 * (year % 100) ) / 4
+ ( 21 * (year / 100) ) / 4 ) % 7;
return w;
}
