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    From:    Kan Yabumoto           tech@xxcopy.com
    To:      XXCOPY user
    Subject: Windows File Date and Time
    Date:    2000-06-07
    File time in DOS
      The good old DOS had just one file date value which keeps track of
      a file in your storage (hard disk and floppy).  To be precise, the
      value has two parts, the date part (year, month, and day) and the
      time part (hour, minute, second ---- measured in two second interval),
      but we will call it just "file date" in this discussion.
      Whenever a file is created, the current system time is stamped to the
      file which would remain constant even if it is copied or moved to a
      new directory.  A complete rewrite, partial rewrite, or partial
      deletion would update the file date value.  Therefore, the DOS file
      date represents the last-write (or, last-modified) time.  It was
      quite simple and well.
    File time in Win32:
      The new so-called Win32 environments (Windows 95, 98, NT, 2000)
      expanded the file date to hold more information about the history of
      the file.  Win32 maintains three distinct time stamps on every file.
      Inside Windows Explorer, you can examine these values in the property
      sheet for a file.
        1. Created:    It is the time when the file is created in the
                       current directory.  When the file is copied to a
                       new directory, a new value will be set.
        2. Modified:   It is the time when the file is last modified.
                       When the file is copied to elsewhere, the same
                       value will be carried over to the new directory.
        3. Accessed:   It is the time when the file is last accessed.
                       This value is set by the application program that
                       sets or revises the value.  Unfortunately some
                       applications do not revise this value.
      The file date value commonly referred to under Win32 is the "Last-
      modified" value (2nd one in the list above) whose behavior is
      consistent to the DOS file date value.  The Win32 file date values
      are stored in much finer resolution than the DOS time stamp (16 bits
      for the date and 16 bits for time).   The Win32 file date value
      is a 64-bit quantity which represents the time elapsed from
      January 1, 1601 (the first date of the current quadri-century)
      in 100 nsec granularity.  For the compatibility's sake, even WinNT/2K
      uses the same 2-second granularity for the "Last-modified" time for
      FAT-based file system (does not apply for NTFS files).
    XXCOPY's file date treatments:
      XXCOPY provides the following switches to select one of the
      three timestamps as the filedate value for time comparison.
         /FC     File-Create time
         /FW     Last-Modify (Last-Write) time (default)
         /FA     Last-Access time
      These switches do not perform any action by themselves.  They
      are used to modify the semantics of other switches which use
      the file date parameters in the file selection process.  For
      example, /DA and /DB are often modified by the /FC switch.
    The file date (Last-Modify date):
      The common file date value (more precisely, the Last-Modify-date)
      is the most intuitive and probably the easiest to use.  So, by
      default, XXCOPY's file date functions use the Last-Modify date by
      default.  For example,
        XXCOPY  c:\mydir\  d:\backup\  /DA#7
      The /DA#7 switch selects files which are last modified within
      the last 7 days.  This selection includes files which are created
      or modified elsewhere and brought to the source directory by
      either a copy or move operation.  The COPY or MOVE operations
      carried out by practically all file copy utilities (i.e.,
      Drag-and-drop, COPY, XCOPY, MOVE, or XXCOPY) preserve the file's
      Last-Modify date.
    The file creation date (File-Create date):
      Another useful date value is the File-Create date.  Unlike the
      Last-Modify date, this value represents the date the particular
      copy of the file is created in the directory.  Here, the meaning
      of creation includes both the case of a newly created file, and an
      existing file brought in to the directory by a copy operation.
      So, the File-Create date is often newer than the Last-Modify date.
      Note that sometimes, the "File-Create" date could be a little
      misleading.  But, in this article we use the "File-Create" date
      consistent with the way Microsoft calls it.
      With XXCOPY, you may use this creation-date value instead of the
      more common Last-Modify date.  Here is an example:
        XXCOPY  c:\mydir\  d:\backup\  /S /FC /DA:.
          This command copies all the files which are either made in
          or brought into their present directory today regardless of
          the age of the file.  With the /FC switch, XXCOPY uses the
          File-Create date rather than the Last-Modify date.  The
          /DA:. switch selects files of today or a future date.
      Since the use of the File-Create date has serious problems, we
      generally discourage the use if this date
    Problems with the file creation date (File-Create date),
      The problems of the File-Create date can be traced back to
      the inconsistency in Microsoft's various file management
      utilities.  It seems that the purpose of three distinct
      variations in the file date values were never clearly defined
      by the designer of the feature.  We as software developers
      have not come across any official documents on this subject.
      So, we conduct a few experiments using Microsoft's programs
      which are part of Windows 95.  Then, you will find many
      inconsistent usages in the File-Create date.
      Observation 1:  When you perform a copy operation of a file
                      which results in a new physical copy in the
                      destination, the File-Create date is set to
                      the current date.
      Observation 2:  When you move a file within a volume, the
                      operation is translated to the more efficient
                      renaming operation.  Since renaming a file does
                      not involve in a newly created file, the File-
                      Create date will not be updated.
      Observation 3:  When you move a file across the volume boundary
                      (e.g., from C: to D:), the move operation is
                      carried out as a file copy action followed by
                      a file delete action, the file in the new
                      location will receive a new File-Create date.
      Observation 4:  Edit a file using either NotePad.exe, WordPad.exe
                      or WinWord.exe (word), and save the file.  The
                      newly update file will have the same File-Create
                      date, but a new Last-Modify time.
      The inconsistencies listed above make the File-Create date unfit
      for a general-purpose file selection criterion by XXCOPY.  On
      the other hand, if you have full control of the file creation
      process in a given directory (say, you always use one of the
      file copy operations to manage files in the directory), you may
      still use it with caution.
    The case with the Last-Access date:
      This parameter is also a very controversial value that goes with
      every file in the Win32 system.  The Last-Access date is set whenever
      the file is "Accessed" by a program.  Then, the next question is
      what really constitutes an "Access" to a file?
      Is opening the file by a program, by any program, treated as an
      "Access"?  Thank God, the answer is no.  If that were the case,
      whenever the Windows Explorer displays an executable file using its
      icon (which is stored inside the file), the Last-Access date would
      be set to the current date.  That is because display of the icon
      involves fist opening the file and reading the contents to locate
      the internal icon.  In this case, although the treatment of the icon
      is rather elaborate under the cover, it is not regarded as an "Access".
      On the other hand with .EXE and .DLL files, executing the program
      constitute the Last-Access.  That makes sense.
      But, there are plenty of silly mistakes committed by Microsoft's
      programmers which makes the Last-Access date of little use.  The
      possibly the worst program mistake with this value is by Windows
      As shown above, when you click the right button on an icon of a file
      and select the properties menu, you can examine the Last-Access date
      (in this case you get only the date without time) along with the other
      two file date values.  But, if you are alert, you will notice that
      the Last-Access date is always today's date.  Yes, the very act of
      examining the Last-Access date value triggers the update of the value.
      That is sad.  Very sad.
      When a system administrator makes a regularly scheduled backup, he
      usually performs a full backup every so often, copying every file
      in a drive.  Now, that is an act of Access.  Copying a file will
      also update the Last-Access date.
      If Microsoft's programmers had been very careful not updating the
      Last-Access values under certain common file access cases, this
      value could have become a very useful parameter in file selection
      for file management (backup) activities.  Unfortunately, they wrote
      so many programs that ignored the designer's apparent intention.
      Now, it is too late.  The Last-Access date is even more useless
      than the doomed File-Create date.
      We recommend the use of /FC (File-Create date) only in carefully
      controlled circumstances.  The Last-Access date (selected by /FA)
      seems nearly useless for meaningful file management activities.
      Original DOS file system had only 32 bytes to represent a file in
      the directory.  The very restrictive 8.3 filename and the limited
      granularity (2 second) in file date are corrected in the Win32
      file systems (VFAT).  But, the generous allocation of lots of
      bytes (24 bytes just to store the file date in three flavors) which
      bloats the system resource usage but provides little useful
      information seems to symbolize what we know as Windows Operating
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