Logic Behind AD to BS and BS to AD date conversion

the difference between BS and AD is:

56 years 8 months 17 days

so here is a simple c program to demonstrate it:

/* Program to convert BS to AD */
#include<stdio.h>
#include<conio.h>

void main()
{
 int year, month, date;
 clrscr();
 /* Taking Input from User */
 printf("Give BS date in year month and date:n");
 scanf("%d%d%d",&year,&month,&date);
/* Changing Date */
if(date > 17)
 {
  date -= 17;
 }
 else
 {
  date += 30;
  --month;
  date -= 17;
 }

 if(month > 8)
 {
  month -= 8;
 }
 else
 {
  month += 12;
  --year;
  month -= 8;
 }
 year -= 56;

 /* Display Output */
printf("Date in AD is : %d-%d-%dn",year, month, date);

 getch();
}

hope you understood if not do comment your confusion

goodluck