Changing the name of the file and adding a few more lines to the code

calendar.c

+/**
+ * File: calendar.c
+ * Author Gurcan Gulec
+ * Created: 19 October 2016
+ * Last Edit: 20 October 2016
+ *
+ * Description: Gregorian calendar thing.
+ *
+ */
+
+#include <stdio.h>
+#include <math.h>
+
+int main()
+{
+
+    int date;
+    int leap = 0;
+    int m1[7] = {1, 3, 5, 7, 8, 10, 12}; 
+
+    scanf("%d", &date);
+    // finding the year
+    int year = date % 10000;
+    
+    // finding the month
+    int month = ((date % 1000000) - year) / 10000;
+
+    // finding the day
+    int day = ((date % 100000000) - month * 10000 - year) / 1000000;
+
+    printf("Day   = %d\n",day);
+    printf("Month = %d\n",month);
+    printf("Year  = %d\n",year);
+
+    // month cannot be bigger than 12
+    if(month > 12)
+    {
+        printf("Date not valid\n");
+        return 0;
+    }
+
+    // checking the months that has 31 days and 30 days and decide if it is valid
+    for(int a = 0; a < 7; a++)
+    {
+        if(month == m1[a])
+        {
+            if(day > 31)
+            {
+                printf("Date not valid\n");
+                return 0;
+            }
+        }
+        else
+        {
+            if(day > 30)
+            {
+                printf("Date not valid\n");
+                return 0;
+            }
+        }
+            
+    }
+
+
+    // deciding leap years
+    if(year % 100 == 0)
+    {
+        if(year % 400 == 0)
+        {
+            leap = 1;
+        }
+        else
+        {
+            leap = 0;
+        }
+    }
+    else
+    {
+       if(year % 4 == 0)
+       {
+           leap = 1;
+       }
+       else
+       {
+           leap = 0;
+       }
+
+    }
+
+    // checking february
+    if(month == 2)
+    {
+
+        if(leap == 1)
+        {
+            if(day != 29)
+            {
+                printf("Date not valid\n");
+                return 0;
+            }
+        }
+        else
+        {
+            if(day != 28)
+            {
+                printf("Date not valid\n");
+                return 0;
+            }
+
+        }
+
+    }
+
+    printf("Date is valid\n");
+    return 0;
+    
+}