// ...

#define _date_t_year_width 23
#define _date_t_month_width 4
#define _date_t_day_width 5

// ...

typedef enum _month_t {
 _month_t_zero = 0, Jan = 1, Feb = 2, Mar = 3, Apr = 4, May = 5, Jun = 6, Jul = 7, Aug = 8, Sep = 9, Oct = 10, Nov = 11, Dec = 12,
} month_t;

typedef struct _date_t {
 unsigned day : _date_t_day_width;
 unsigned month : _date_t_month_width;
 signed  year : _date_t_year_width;
} date_t;

// ...

template <class T> T inline Abs( T n ) {
 if ( n < 0 ) return -n; return n;
}

// ...

inline unsigned char DaysInMonth( month_t month, unsigned long int year ) {
 switch ( month ) {
 case Jan: return 31;
 case Feb: if ( ( ( unsigned long int )year % 4 ) ) return 28; return 29;
 case Mar: return 31;
 case Apr: return 30;
 case May: return 31;
 case Jun: return 30;
 case Jul: return 31;
 case Aug: return 31;
 case Sep: return 30;
 case Oct: return 31;
 case Nov: return 30;
 case Dec: return 31;
 default: return 0;
 }
}

inline unsigned short int DaysToMonth( month_t month, unsigned long int year ) {
 unsigned short int result = 0;
 for ( int i = Jan; i < month; i++ ) result += DaysInMonth( ( month_t )i, year );
 return result;
}

// ...

inline bool date_isvalid( signed long int year, unsigned long int month, unsigned long int day ) {
 if ( year && month && day
  && Abs( year ) < ( 1 << ( _date_t_year_width - 1 ) )
  && month < 13 && day <= DaysInMonth( ( month_t )month, ( unsigned ) Abs( year ) )
 ) return true;
 return false;
}