C / C++ / MFC

/* ***************************************** */
/* Parameters:                               */
/*     int nCurrentDay - the current day     */
/*                                           */
/* Returns:                                  */
/*     1-31 representing yesterday's ordinal */
/*     0 for error                           */
/* ***************************************** */
int YourClass::GetYesterday(int nCurrentDay)
{
   // Error check
   if(nCurrentDay < 1) || (nCurrentDay > 31))
      return 0;

   if(nCurrentDay == 1)
   {
      // Get the current month
      int month = COleDateTime::GetMonth();

      // subtract 1 to get last month
      int lastmonth = month - 1;

      // if you are at zero (meaning month is January) then make lastmonth December
      if(!lastmonth)
         lastmonth = 12;

      // since there is a workable number of months and they are unchanging, hardcode the switch
      switch(lastmonth)
      {
         case 4  :
         case 6  :
         case 10 :
         case 11 : return(30);
         break;


         case 1  :
         case 3  :
         case 5  :
         case 7  :
         case 8  :
         case 9  :
         case 12 : return(31);
         break;


         // for February - do leap year processing. See link for algorithm
         case 2  : 
         if((!COleDateTime::GetYear()%4) && ((COleDateTime::GetYear()%100) || (!COleDateTime::GetYear()% 400))) 
            return(29);
         else
            return(28);
      
      }
   }
   else
      return(nCurrentDay--);
}

COleDateTime dt = COleDateTime::GetCurrentTime();
int today = dt.GetDay();
COleDateTimeSpan span(1.0f);  // One day
dt -= span;
int yesterday = dt.GetDay();

void CMyDlg::GetYesterday() 
{
        // work with the structure SYSTEMTIME to
        // get the local time (which is today)
	SYSTEMTIME st;	
	GetLocalTime(&st);

        // to manipulate time, always convert the SYSTEMTIME to FILETIME
	FILETIME ft;
	SystemTimeToFileTime(&st,&ft);

        // to perform arithmetics actions on time, copy the FILETIME into
        // a ULARGE_INTEGER
	__int64 longTime;
	memcpy(&longTime,&ft,sizeof(longTime));

	// calculate your days as follows:
	// 10000000*60*60*24 = 1 day
	
        // decrease local time with 1 day
	longTime -= (__int64)10000000*60*60*24;
	
        // copy the result back to FILETIME
	memcpy(&ft,&longTime,sizeof(ft));

        // convert FILETIME back to SYSTEMTIME
	FileTimeToSystemTime(&ft,&st);

       // now SYSTEMTIME st holds yesterday 
}