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Last accessed time and last modified time reported by C Run-Time (CRT) functions can be adjusted by using the "Automatically Adjust for Daylight Saving Time" option

This article was previously published under Q190315
You can adjust the time of last access and time of last modificationreported by the CRT functions _stat, _fstat, _findfirst, and _findnext fordaylight saving time under the NTFS file system. This manifests itself asa change in the time last modified when making the switch to daylightsavings time from standard time or vice-versa.
All of the previously referenced CRT functions call the Win32 APIFileTimeToLocalFileTime. FileTimeToLocalFileTime adjusts the file times fordaylight saving time if "Automatically adjust clock for daylight savingchanges" is selected in the Date/Time Properties dialog box for the systemclock. This behavior is by design under the NTFS file system.
One way to avoid the behavior is to clear "Automatically adjust clock fordaylight saving changes" in the Date/Time Properties dialog box for thesystem clock. Other workarounds depend upon the calculations or assumptionsthat are made based on the date/time last modified that is reported for afile.

Note If the time last modified is between 12:00 am and 1:00 am, thedate last modified is also changed when the adjustment subtracts an hourfrom the time last modified in the Fall. Similarly, files with a time lastmodified between 11:00 pm and midnight will have their date last modifiedwhen the adjustment advances the time one hour in the Spring.
This behavior is by design.
This behavior is specific to the NTFS file system, and does not reproduceon a system using the FAT file system. Note that the CRT and the operatingsystem report the same time last modified under all circumstances.

In the steps below, references to stat.exe mean the program generated bycompiling stat.c, which appears below:

Sample code

   /* STAT.C: This program uses the _stat function to    * report information about the file named STAT.C.    * Compile options: none.    */    #include <time.h>   #include <sys/types.h>   #include <sys/stat.h>   #include <stdio.h>   int main(int ac, char **av)   {      struct _stat buf;      if(ac != 2)      {         printf("Usage: %s <FileName>\n",av[0]);         return -1;      }      /* Get File Statistics for stat.c. */       if( _stat( av[1], &buf ) != 0 )         perror( "Problem getting information" );      else /* print the date/time last modified */          printf( "Date/Time modified : %s", ctime( &buf.st_mtime ) );   }   /* End stat.c. */ 				

Steps to reproduce the behavior (must be an NTFS drive)

  1. Open the Date/Time Properties dialog box and ensure that "Automatically Adjust For Daylight Saving Time" is selected. Press OK.
  2. Set your system date to a date that is during daylight saving time (for example, 10/25/97 in the "Pacific Time (US & Canada); Tijuana" time zone).
  3. Create a new file called repro.txt.
  4. Note the date/time last modified for repro.txt, as reported by either the Windows Explorer or using an MS-DOS dir command.
  5. At the MS-DOS prompt, run "c:\>stat.exe repro.txt", and note that the date/time last modified matches the date/time last modified that the system reports.
  6. Reset your system date to a date that is in the standard time period.
  7. Look again at an MS-DOS dir command (or in the Windows Explorer) and you can see that the time last modified for repro.txt is now one hour earlier than before.
  8. Repeat step 5, and note that the date/time reported by stat.exe is also one hour earlier, matching what the operating system reports.
  9. Clear "Automatically Adjust for Daylight Saving Time."
  10. Repeat steps 7 and 8, noting that the adjustment is not being made, and the time last modified has returned to its original value.
A similar series of steps can be followed to observe the time beingadjusted forward one hour by creating the file during standard time andreferencing it after the switch to daylight saving time.
_stat, _fstat, _findfirst, _findnext, NTFS

Article ID: 190315 - Last Review: 01/23/2007 19:29:25 - Revision: 3.1

  • The C Run-Time (CRT)
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