C Program to find day of week given date

Question

Is there a way to find out day of the week given date in just one line of C code?

For example

Given 19-05-2011(dd-mm-yyyy) gives me Thursday

Answer 1

For Day of Week, years 2000 - 2099.

uint8_t rtc_DayOfWeek(uint8_t year, uint8_t month, uint8_t day)
{
    //static const uint8_t month_offset_table[] = {0, 3, 3, 6, 1, 4, 6, 2, 5, 0, 3, 5}; // Typical table.

    // Added 1 to Jan, Feb. Subtracted 1 from each instead of adding 6 in calc below.

    static const uint8_t month_offset_table[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};

    // Year is 0 - 99, representing years 2000 - 2099

    // Month starts at 0.

    // Day starts at 1.

    // Subtract 1 in calc for Jan, Feb, only in leap years.
    // Subtracting 1 from year has the effect of subtracting 2 in leap years, subtracting 1 otherwise.
    // Adding 1 for Jan, Feb in Month Table so calc ends up subtracting 1 for Jan, Feb, only in leap years.
    // All of this complication to avoid the check if it is a leap year.
    if (month < 2) {
        year--;
    }

    // Century constant is 6. Subtract 1 from Month Table, so difference is 7.

    // Sunday (0), Monday (1) ...

    return (day + month_offset_table[month] + year + (year >> 2)) % 7;

} /* end rtc_DayOfWeek() */