tarih proğramdaki mantık

C proğramlamda tarihi kullanıcıdan alan ve günü gösteren proğramın kodu aşagıdaki gibidir. Kırmızı renkle gösterilen kısımların mantığını açıklar mısınız?:-)

#include<stdio.h>
#include<conio.h>
#include<math.h>

int fm(int date, int month, int year) {
int fmonth, leap;

//leap function 1 for leap & 0 for non-leap
if ((year % 100 == 0) && (year % 400 != 0))
leap = 0;
else if (year % 4 == 0)
leap = 1;
else
leap = 0;

fmonth = 3 + (2 - leap) * ((month + 2) / (2 * month))
+ (5 * month + month / 9) / 2;

//bring it in range of 0 to 6
fmonth = fmonth % 7;

return fmonth;
}

//----------------------------------------------
int day_of_week(int date, int month, int year) {

int dayOfWeek;
int YY = year % 100;
int century = year / 100;

printf("\nDate: %d/%d/%d \n", date, month, year);

dayOfWeek = 1.25 * YY + fm(date, month, year) + date - 2 * (century % 4);

//remainder on division by 7
dayOfWeek = dayOfWeek % 7;

switch (dayOfWeek) {
case 0:
printf("weekday = Saturday");
break;
case 1:
printf("weekday = Sunday");
break;
case 2:
printf("weekday = Monday");
break;
case 3:
printf("weekday = Tuesday");
break;
case 4:
printf("weekday = Wednesday");
break;
case 5:
printf("weekday = Thursday");
break;
case 6:
printf("weekday = Friday");
break;
default:
printf("Incorrect data");
}
return 0;
}
//------------------------------------------
int main() {
int date, month, year;

printf("\nEnter the year ");
scanf("%d", &year);

printf("\nEnter the month ");
scanf("%d", &month);

printf("\nEnter the date ");
scanf("%d", &date);

day_of_week(date, month, year);

return 0;
}