Calculate the local day of the week from Unix time and time zone offset

dayofweek.c

#include <stdio.h>
#include <time.h>

// Calculate the current day of the week as an integer
//   now - Unix timestamp like that from time(NULL)
//   tz_offset - Number of hours off from UTC; i.e. PST = -8
//   Return value: Sunday=0, Monday=1, ... Saturday=6
int dayofweek(time_t now, int tz_offset) {
	// Calculate number of seconds since midnight 1 Jan 1970 local time
	time_t localtime = now + (tz_offset * 60 * 60);
	// Convert to number of days since 1 Jan 1970
	int days_since_epoch = localtime / 86400;
	// 1 Jan 1970 was a Thursday, so add 4 so Sunday is day 0, and mod 7
	int day_of_week = (days_since_epoch + 4) % 7; 

	return day_of_week;
}

int main() {
	time_t now = time(NULL);

	printf("Current day of week: %i\n", dayofweek(now, -7));
}