FileTimeToLocalFileTime() Adjusts for Daylight Saving Time

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT version 3.5
  

SUMMARY

Under NTFS, the API GetFileTime() returns the create time, last access time, and last write time for the specified file. The times returned in the FILETIME structures are in Universal Coordinated Time (UTC). This is also the time that NTFS uses. You can use FileTimeToLocalFileTime() to convert a file time to a local time. However, if you automatically adjust for Daylight Saving Time, FileTimeToLocalFileTime() will adjust for Daylight Saving Time based on whether the current date should be adjusted for Daylight Saving Time, not based on whether the date represented by the FILETIME structure should be adjusted.

The behavior in this situation is different under FAT, but may be changed to match the behavior under NTFS in a future version of Windows NT.

MORE INFORMATION

The result of this behavior, which is by design, is that reported file times under NTFS may change with the start and end of Daylight Saving Time. For example, suppose that the file TEST.C has a last write FILETIME representing Jan 1, 1995 9:00pm (UTC), it is not Daylight Saving Time, and you are in the Pacific time zone. Both the DIR command and the following sample code report the file time as 1:00pm (LocalTime = UTC - 8).

Sample Code 1

   #include <windows.h>

   void main()
   {
      HANDLE hFile;
      FILETIME ftCreate, ftLastAccess, ftLastWrite, ftLocal;
      SYSTEMTIME st;

      char buf[80];

   // Open the file.

      hFile = CreateFile( "test.c",
                          GENERIC_READ,
                          0,
                          NULL,
                          OPEN_EXISTING,
                          FILE_ATTRIBUTE_NORMAL,
                          NULL );

   // Get the file time (in UTC) and convert to local time.

      GetFileTime( hFile, &ftCreate, &ftLastAccess, &ftLastWrite );
      FileTimeToLocalFileTime( &ftLastWrite, &ftLocal );

   // Display the time, as a test.

      FileTimeToSystemTime( &ftLocal, &st );
      GetTimeFormat( LOCALE_USER_DEFAULT, 0, &st, NULL, buf, sizeof(buf) );
      MessageBox( NULL, buf, " FILE TIME", MB_OK );
   }

The following sample code will correctly report the file time of TEST.C with the date set to 7/1/95 under NTFS. The FILETIME structure is converted to a SYSTEMTIME structure with FileTimeToSystemTime(). Then the time is converted using SystemTimeToTzSpecificLocalTime(). If you need to convert back to a FILETIME structure, use SystemTimeToFileTime() after the conversion to local time.

Sample Code 2

   #include <windows.h>

   void main()
   {
      HANDLE hFile;
      FILETIME ftCreate, ftLastAccess, ftLastWrite;
      SYSTEMTIME stUTC, st;
      char buf[80];

   // Open the file.

      hFile = CreateFile( "test.c",
                          GENERIC_READ,
                          0,
                          NULL,
                          OPEN_EXISTING,
                          FILE_ATTRIBUTE_NORMAL,
                          NULL );

   // Get the file time (in UTC) and convert to local time.

      GetFileTime( hFile, &ftCreate, &ftLastAccess, &ftLastWrite );
      FileTimeToSystemTime( &ftLastWrite, &stUTC );
      SystemTimeToTzSpecificLocalTime( NULL, &stUTC, &st);

   // Display the time, as a test.

      GetTimeFormat( LOCALE_USER_DEFAULT, 0, &st, NULL, buf, sizeof(buf) );
      MessageBox( NULL, buf, "FILE TIME", MB_OK );
   }

On the other hand, FindFirstFile() on FAT always reads the time from the file (stored as local time) and converts it into UTC, adjusting for DST based on the current date. So if FindFirstFile() is called, the date is changed to a different DST season, and then FindFirstFile() is called again, the UTC returned by the two calls will be different.