Find day of the week for a given date | GeeksforGeeks

Write a function that calculates the day of the week for any particular date in the past or future. A typical application is to calculate the day of the week on which someone was born or some other special event occurred.

int dayofweek(int d, int m, int y)

{

    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;

}

http://en.wikipedia.org/wiki/Determination_of_the_day_of_the_week
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