static int day_tab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
} ;

daytodate_(year,yearday, month, day) /* set month, day */
int *year, *yearday, *month, *day ;
{
   int i, leap ;

   leap = *year%4 == 0 && *year%100 != 0 || *year%400 == 0 ;
   for (i = 1; *yearday > day_tab[leap][i] ; i++)
       *yearday -= day_tab[leap][i] ;
   *month = i ;
   *day = *yearday ;
}

datetoday(year,month,day,yearday)   /* set yearday */
int *year, *month, *day, *yearday ;
{
   int i, leap ;
   leap = *year%4 == 0 && *year%100 != 0 || *year%400 == 0 ;
   *yearday = *day ;
    for (i = 1; i < *month ; i++ ) 
        *yearday += day_tab[leap][i] ;
}