PRB: CTimeSpan::GetDays() Reports 29 Days in April

• The Microsoft Foundation Classes (MFC) included with:

      - Microsoft C/C++ version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0, 1.5, 1.51, 1.52
      - Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0
  

SYMPTOMS

The CTimeSpan::GetDays function seems to incorrectly report that the number of days between April 1 and May 1 is 29, instead of 30.

CAUSE

This behavior of the CTimeSpan::GetDays function is by design. One day in April is the start of Daylight Savings Time. On that day, there are only 23 hours, instead of the normal 24. Because of the way the CTimeSpan::GetDays member function is implemented, a day with less than 24 hours is not a complete day.

Because the time span is figured in seconds, the time span for a month is the sum of all the seconds in a month. The definition of CTimeSpan::GetDays() in AFX.INL (located in the MFC\INCLUDE sub-directory of the Visual C++ installation directory)calculates the number of days by dividing the value of the m_timeSpan member variable by the number of seconds in a day. Thus, if one day in a 30-day month has 23 hours (which is the case here), m_timeSpan will be short by 3600 seconds, and the result of the division will be 29 (days), rather than 30. This is a characteristic of integer division in C/C++ where the remainder is truncated.

RESOLUTION

There are two workarounds:

• If you are interested only in the precision of the number of days, add 3600 seconds to the instance of CTimeSpan. This ensures that the April case is handled appropriately, and that other calculations will not be affected, as a characteristic of integer division. Adding 3600 seconds (one hour) to a CTimeSpan instance will not affect measurement in days even in the worst case of a time span being some multiple of 86400 seconds (one day), because a day with one hour is not a full day.

  -or-

• Use the TZ environment variable to control how time calculations are handled by MFC. The TZ environment variable can be set from the command line (such as in AUTOEXEC.BAT), or it can be set with the _putenv() function. The documentation for the TZ environment variable [in the online help or the "Run-Time Library Reference," under _tzset()], states that the default value of TZ is PST8PDT, which corresponds to the Pacific time zone.

  To work around the problem at hand, just set ONLY the tzn (three letter time zone) and hh (difference in hours between Universal Coordinated Time/Greenwich Mean Time) fields of the TZ variable. For example, if you are in the Eastern Time Zone, and set the TZ variable to EST5, then Daylight Savings Time would not be taken into account when doing CTime and CTimeSpan calculations.

  NOTE: You MUST set the TZ variable before doing ANY time calculations, including initialization of CTime objects. Try moving the _putenv() call in the following code to demonstrate this. A good place to set the TZ environment variable is in the application's InitInstance() function.

Sample Code