Calendar Application using Data Structures

We trying to develop the Calendar application’s functional part only which is called Algorithm or Data Structure.

int isLeapYear( int yr );        
int leapYears( int yr );         
int todayOf( int y, int m, int d); 
long days( int y, int m, int d);   
void calendar(int y, int m);       
int getDayNumber(int d,int m,int y);
char *getName(int day);
void flush()
{
    int c;
    while ((c = getchar()) != '\n' && c != EOF);
}
 
typedef struct {
  int day;
  int month;
  int yr;
  char note[255];
} Note;
 
int main(int argc, char* argv[]){
    int yr, month, day;
    char choice;
    Note note;
    FILE *fp;
 
    fp = fopen("note.bin", "r");
    if (fp == NULL) {
      fp = fopen("note.bin", "w");
    } 
    fclose(fp);
 
    while(1) {
      printf("1. Find the day\n");
      printf("2. Print calendar of a month\n");
      printf("3. Add Note\n");
      printf("4. Exit\n");
      printf("Enter your choice: ");
      scanf("\n%c", &choice);
      switch(choice) {
        case '1':
        printf("Enter the day, month and year: ");
        scanf("%d %d %d", &day, &month, &yr);
        printf("The day is : %s\n", getName(getDayNumber(day, month, yr)));
        break;
        case '2':
        printf("Enter the month and year: ");
        scanf("%d %d", &month, &yr);
        printf("Please enter 's' to see the notes\n Press any other key to continue\n");
        calendar(yr, month);
        break;
        case '3':
        printf("Enter the day, month and year: ");
        scanf("%d %d %d", &note.day, &note.month, &note.yr);
        flush();
        printf("Enter the note: ");
        fgets(note.note, 255, stdin);
        fp = fopen("note.bin", "a+");
        if (fp == NULL) {
          printf("File note.bin can not be opened\n");
          exit(1);
        }
        fwrite(&note, sizeof(Note), 1, fp);
        printf("Note added sucessfully\n");
        fclose(fp);
        break;
        case '4':
        printf("Bye!!");
        exit(0);
        break;
        default:
        printf("Not a valid option\n");
        break;
      }
    }
    return 0;
}
 
int isLeapYear( int y ){
    return(y % 400 == 0) || ((y % 4 == 0) && (y % 100 != 0));
}
 
int leapYears( int y ){
    return y/4 - y/100 + y/400;
}
 
int todayOf( int y, int m, int d) {
    static int DayOfMonth[] = 
        { -1,0,31,59,90,120,151,181,212,243,273,304,334};
    return DayOfMonth[m] + d + ((m>2 && isLeapYear(y))? 1 : 0);
}
 
long days( int y, int m, int d){
    int lastYear;
    lastYear = y - 1;
    return 365L * lastYear + leapYears(lastYear) + todayOf(y,m,d);
}
 
void calendar(int y, int m){
    FILE *fp;
    Note* notes, note;
    int len, j, hasNote = 0;
    char choice;
    const char *NameOfMonth[] = { NULL/*dummp*/,
        "January", "February", "March", "April", "May", "June",
        "July", "August", "September", "October", "November", "December"
    };
    char Week[] = "Su   Mo   Tu   We   Th   Fr   Sa";
    int DayOfMonth[] =
        { -1,31,28,31,30,31,30,31,31,30,31,30,31 };
    int weekOfTopDay;
    int i,day;
 
    weekOfTopDay = days(y, m, 1) % 7;
 
    fp = fopen("note.bin", "rb");
    if (fp == NULL) {
      printf("Couldn't read notes\n");
    }
    len = 0;
    while(fread(&note, sizeof(Note), 1, fp)) {
      if (note.yr == y && note.month == m) {
        len++;
      }
    }
    rewind(fp);
    j = 0;
    notes = (Note*) malloc (sizeof(Note) * len);
    while(fread(&note, sizeof(Note), 1, fp)) {
      if (note.yr == y && note.month == m) {
        notes[j] = note;
        j++;
      }
    }
 
    fclose(fp);
 
    if(isLeapYear(y))
        DayOfMonth[2] = 29;
    printf("\n     %s %d\n%s\n", NameOfMonth[m], y, Week);
 
    for(i=0;i<weekOfTopDay;i++)
        printf("   ");
    for(i=weekOfTopDay,day=1;day <= DayOfMonth[m];i++,day++){
        hasNote = 0;
        for (j = 0; j < len; j++) {
          if (notes[j].day == day) {
            printf("|%2d| ",day);
            hasNote = 1;
            break;
          }
        }
        if (hasNote == 0) {
          printf("%2d   ",day);
        }
        if(i % 7 == 6)
            printf("\n");
    }   
    printf("\n");
    scanf("\n%c", &choice);
    if (choice == 's') {
      printf("Here are list of notes for %d %d\n", m, y);
      for (j = 0; j < len; j++) {
        printf("%d: %s\n", notes[j].day, notes[j].note);
      }
    } else {
      return;
    }
}
 
int getDayNumber(int d, int m, int y){ 
    static int t[] = {0, 3, 2, 5, 0, 3, 5, 1, 4, 6, 2, 4};
    y -= m < 3;
    return (y + y/4 - y/100 + y/400 + t[m-1] + d) % 7;
}
 
char *getName(int day){ 
   switch(day){
      case 0 :return("Sunday");
      case 1 :return("Monday");
      case 2 :return("Tuesday");
      case 3 :return("Wednesday");
      case 4 :return("Thursday");
      case 5 :return("Friday");
      case 6 :return("Saturday");
      default:return("Error: Invalid Argument Passed");
   }
}