FIX: COleDateTimeSpan Returns Incorrect Value

• The Microsoft Foundation Classes (MFC), used with:
  • Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1

SYMPTOMS

COleDateTimeSpan returns an incorrect number of days when using dates earlier than midnight December 30, 1899. Also, adding or subtracting COleDateTimeSpan objects from COleDateTime objects will produce incorrect results if the resulting date is before midnight December 30, 1899.

CAUSE

COleDateTime stores a double that represents the number of days relative to midnight December 30, 1899 (the base date). A date prior to the base date is represented by a negative number. A date that falls after the base date is represented by a positive double value. For example, if the date is December 29, 1899 at midnight, the value of COleDateTime is -1.0.

To calculate the actual number of days between a date prior to the base date and the base date, add the fractional part of the COleDateTime's double variable instead of subtracting it. For example, December 29, 1899 at 6:00 a.m. is represented as -1.25. Note, it is .75 days from the base date of midnight December 30, 1899.

The DoubleFromDate() and DateFromDouble() in OLEVAR.CPP located in the \Msdev\Mfc\Src directory attempt to correct for the difference with negative numbers as mentioned above. These functions incorrectly handle this.

Sample Code

   double DoubleFromDate(DATE dt)
   {

       // No problem if it's positive.
       if(dt >= 0)
           return dt;

       // If it's negative, must convert because negative dates not
       // continuous (for example, -1.25 to -.75, -1.50 to -.50,
       // -1.75 to -.25).
       double dblWhole = modf(dt, &dt);    // dt is now a fractional part.
       return dblWhole - dt;
   }

   DATE DateFromDouble(double dbl)
   {

     // No problem if it's positive.
     if(dbl >= 0)
        return dbl;

     // If it's negative, must convert because negative dates not
     // continuous (for example, -.75 to -1.25, -.50 to -1.50,
     // -.25 to -1.75).
     DATE dtWhole = modf(dbl, &dbl); // dbl is now a fractional part.
     return dtWhole - dbl;
   } 

RESOLUTION

You can use one of the following ways to work around these bugs:

• Create your own COleDateTimeSpan object to handle this correctly. Use the COleDateTimeSpan code in Olevar.cpp as a guide. Then you can cut and paste. Unfortunately, no virtual functions exist for COleDateTimeSpan that will allow you to easily correct the bug.



• Write functions that will produce the same results as COleTimeSpan. See the sample code in this article for a function that will calculate the number of days between two given dates. The function always returns a positive number of days to avoid any confusion about what negative and positive time spans mean. There is also a function that allows you to add a number of days to an existing COleDateTime object.

STATUS

This bug was corrected in Visual C++, 32-bit Edition, version 4.2.

MORE INFORMATION

Sample Code

   #include <afxole.h>
   #include <math.h>

   double GetDaySpan(COleDateTime dt1, COleDateTime dt2)
   {

       // No problem if it's positive.
       if(dt1>=0 && dt2 >= 0)
              return fabs(dt1-dt2);

       // Days from midnight December 30, 1899.
       double Dt1SpanFromZero, Dt2SpanFromZero;

       if (dt1<0)
       {
           double dblDt1Whole, dblDt1Fraction;
           dblDt1Fraction=modf(dt1,&dblDt1Whole);
           Dt1SpanFromZero=dblDt1Whole-dblDt1Fraction;
       }
       else
       {
           Dt1SpanFromZero=dt1;
       }

       if (dt2<0)
       {
           double dblDt2Whole,dblDt2Fraction;
           dblDt2Fraction = modf(dt2, &dblDt2Whole);
           Dt2SpanFromZero=dblDt2Whole-dblDt2Fraction;
       }
       else
       {
           Dt2SpanFromZero=dt2;
       }

       // Return positive number of days.
       return fabs(Dt1SpanFromZero-Dt2SpanFromZero);
   }

   void AddDays(COleDateTime &date, double dblDays)
   {
       // First, normalize the negative date.
       // Days from midnight December 30, 1899.
       double Dt1SpanFromZero;

       if (date.m_dt<0)
       {
           double dblDt1Whole, dblDt1Fraction;
           dblDt1Fraction=modf(date.m_dt,&dblDt1Whole);
           Dt1SpanFromZero=dblDt1Whole-dblDt1Fraction;
       }
       else
       {
           Dt1SpanFromZero = date.m_dt;
       }

        date.m_dt = Dt1SpanFromZero + dblDays;

        if (date.m_dt>=0)
                 return;

        // Date before midnight December 30, 1899.
        // You need to correct the date by changing the fractional portion.

        double dblWhole, dblFraction;
        dblFraction=modf(date,&dblWhole);
        date.m_dt=dblWhole -  ( 2 + dblFraction);
        // Add 1 to make the fraction positive.
        // Add 1 more to increase the whole part by one.
        // For example, it converts a span of  -1.75 (which is -1 - .75 )
        // to -2.25 ( which is -2 + .25 )

   } 

Additional query words:

Keywords : kbole kbMFC kbVC
Version : winnt:4.0,4.1
Platform : winnt
Issue type : kbbug