FIX: COleDateTimeSpan Returns Incorrect Value

The information in this article applies to:

• The Microsoft Foundation Classes (MFC) included with: Microsoft Visual C++, 32-bit Edition, 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.

Here is the code:

double DoubleFromDate(DATE dt) {

    // No problem if positive
    if(dt >= 0)
        return dt;

    // If negative, must convert because negative dates not continuous
    // (examples: -1.25 to -.75, -1.50 to -.50, -1.75 to -.25)
    double dblWhole = modf(dt, &dt);    // dt is now fractional part
    return dblWhole - dt;

DATE DateFromDouble(double dbl) {

  // No problem if positive
  if(dbl >= 0)
     return dbl;

  // If negative, must convert because negative dates not continuous
  // (examples: -.75 to -1.25, -.50 to -1.50, -.25 to -1.75)
  DATE dtWhole = modf(dbl, &dbl); // dbl is now fractional part
  return dtWhole - dbl;

Notice that the modf() function is called incorrectly. The whole number and the fractional portion are reversed.

RESOLUTION

There are two primary 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.

  -or-

• 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

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem was corrected in Visual C++, 32-bit Edition, version 4.2.

MORE INFORMATION

Sample Code

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

    // No problem if 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 # of days
    return fabs(Dt1SpanFromZero-Dt2SpanFromZero);

void AddDays(COleDateTime &date, double dblDays)

    // First, normalize 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
     // need to correct 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
     // e.g., converts a span of  -1.75 ( which is -1 - .75 )
     // to -2.25 ( which is -2 + .25 )