C语言实现日期计算器：输入MMDDYYYY格式转换与验证

#include "stdio.h"         /* Required for MS-DOS use */
#define ENTER    0x1C0D   /* Enter key */
int year, month, day;
static char *days[8] = {"          ","Sunday    ","Monday       ","Tuesday   ",
                         "Wednesday","Thursday ","Friday    ","Saturday "};
struct TIMEDATE {
         int year;        /* year 1980..2099               */
         int month;       /* month 1=Jan 2=Feb, etc.       */
         int day;         /* day of month 0..31            */
         int hours;       /* hour 0..23                    */
         int minutes;     /* minute 0..59                  */
         int seconds;     /* second 0..59                  */
         int hsecs;       /* 1/100ths of second 0..99      */
         char dateline[47];       /* date & time together */
         };
static struct TIMEDATE today;
main()
{
   char cmonth[3];
   char cday[3];
   char cyear[5];
   double getdays();
   double daynumb, numbnow;
   int weekday, retcode, dayer, i;
   dayer = datetime(&today);
   clrscn();
   for (i=0;i<3;++i)cmonth[i]='\0';
   for (i=0;i<3;++i)cday[i]='\0';
   for (i=0;i<5;++i)cyear[i]='\0';
   putstr(5,8,14,"Enter date in MM DD YYYY format:");

   while (retcode != ENTER)
     {
      retcode = bufinp(5,41,13,2,cmonth);
      if (retcode != ENTER) retcode = bufinp(5,44,13,2,cday);
      if (retcode != ENTER) retcode = bufinp(5,47,13,4,cyear);
     }
   year = atoi(&cyear);
   month = atoi(&cmonth);
   day = atoi(&cday);
   daynumb = getdays(year, month, day);
   numbnow = getdays(today.year, today.month, today.day);
   weekday = weekdays(daynumb);
   if (numbnow - daynumb == 0)
       printf("\n\n%02d-%02d-%d is",month, day, year);
   if (numbnow - daynumb > 0)
       printf("\n\n%02d-%02d-%d was",month, day, year);
   if (numbnow - daynumb < 0)
       printf("\n\n%02d-%02d-%d will be",month, day, year);
   printf(" a %s\n",days[weekday]);
}   /* end MAIN */
/************************************************************
* GETDAYS   - From integer values of year (YYYY), month      *
*             (MM) and day (DD) this subroutine returns a    *
*             double float number which represents the       *
*             number of days since Jan 1, 1980 (day 1).      *
*             This routine is the opposite of GETDATE.       *
************************************************************/
double getdays(year, month, day)
   int year, month, day;
   {