Get weekday from a date, C version

weekday.c

/**
 * Weekday calculation utils.
 *
 * Month and weekday are 1-based, 1 is Monday. 
 * The enums may be replaced by ints at the expense of clarity.
 *
 * Released to the public domain
 */

#include <stdint.h>
#include <stdbool.h>

// Start year for weekday computation 
// - reduces loop iteration count if we know older years won't be needed
#define DT_START_YEAR 2019
// Weekday on January 1st of DT_START_YEAR
#define DT_START_WKDAY TUESDAY

enum weekday {
    MONDAY = 1,
    TUESDAY,
    WEDNESDAY,
    THURSDAY,
    FRIDAY,
    SATURDAY,
    SUNDAY
};

enum month {
    JANUARY = 1,
    FEBRUARY,
    MARCH,
    APRIL,
    MAY,
    JUNE,
    JULY,
    AUGUST,
    SEPTEMBER,
    OCTOBER,
    NOVEMBER,
    DECEMBER
};

/** Days from Jan 1st to the 1st of a month */
static const uint16_t MONTH_YEARDAYS[] = {
    [JANUARY]=0, 31, 59, 90, 120, 151, 181, 212, 243, 273, 304, 334
};

/** Check if a year is leap */
static inline bool is_leap_year(int year)
{
    return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}

/** Get weekday */
enum weekday date_weekday(uint16_t year, enum month month, uint8_t day)
{
    uint16_t i;
    uint16_t days = (DT_START_WKDAY - MONDAY) + (year - DT_START_YEAR) * 365 + MONTH_YEARDAYS[month] + (day - 1);

    for (i = DT_START_YEAR; i <= year; i++) {
        if (is_leap_year(i) && (i < year || month > FEBRUARY)) days++;
    }

    return MONDAY + days % 7;
}