由一个日期, 怎样知道是星期几？

    int dayofweek(int y, int m, int d)  /* 0 = Sunday */    {    static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};    y -= m < 3;    return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;    }