int daysInMonth(int month, int year){
  switch(month){
    case 1: case 3: case 5: case 7: case 8: case 10: case 12: 
      return 31; 
    case 4: case 6: case 9: case 11: 
      return 30; 
    case 2:
      if (isLeapYear(year))
        return 29; 
      else
        return 28;
    default:
      return ERROR_VALUE;
  }
}