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Number   :  9108
Date     :  2004-11-02
Author   :  Garry Deane
Subject  :  Re: Unexpected results all of a sudden
Size(KB) :  2



--- In xxcopy@yahoogroups.com, "John Zeman"  wrote:
> 
> I don't know Des. I didn't realize Windows would manipulate
> the time stamps when daylight savings time changed until I
> noticed the same thing Jim did yesterday regarding xxcopy.
> A side by side time stamp comparison of the C: source files
> and external USB destination files revealed the answer.
> Since /CLONE (and /BU) use the /BI option which compares
> files by size and date, xxcopy saw the newly changed date
> of the source files as being different and just did what
> it was supposed to do.
> 
> Maybe next spring (if I don't forget) I'll disable the
> automatic time change feature and see what happens.
> In the meantime, I'm leaving my fuzzy file time option
> set at /FF3602S
> 
> John

The source of your problem is that one set of files are on
a FAT volume and the other set are on an NTFS volume. The
timestamp of a file on a FAT volume is recorded using local
time. The timestamp of a file on an NTFS volume is recorded
using UTC time.

When you change the regional setting to a different time zone
or daylight savings starts or ends, the timestamp of files on
either volume DOES NOT change. However the timestamp of files
on an NTFS volume APPEARS to change. When windows retrieves
the timestamp for NTFS files and passes that time to the
calling program, it adjusts the UTC time to reflect the local
time zone and/or daylight savings. Therefore although the UTC
time is unchanged, Explorer or Cmd or Xxcopy or whatever
generally uses/displays the time using local time and this
appears to show an altered timestamp.

This wouldn't cause a problem if comparing files between two
different NTFS volumes because the time adjustment would be
the same for both the src and dst. The problem arises when
comparing files on a FAT volume because these times are
stored using local time. Windows makes no time zone
adjustment for these files so their timestamps both remain
the same AND appear the same irrespective of timezone/
daylight savings changes. However when the file timestamps
are compared to the files on a FAT volume, they appear to
have different timestamps.

There isn't really any good solution to this. If you disable
daylight savings adjustment and/or change the system clock
whenever daylight savings starts/ends, the PC is going to
appear to the user to have an incorrect time. Also on
OS's like XP that use time servers by default, it can
actually be quite a challenge to deliberately maintain
the system with the "incorrect" time.

The easiest solution is to do as John suggests and use a
larger fuzzy time (bearing in mind that files altered within
the last hour won't get copied). If you have a predictable
environment such as backing up to an external drive, you
could possibly use a batch file and the various /TT, /TD-
or /TS+ or even /FU to make the necessary adjustments but
I'm not sure that it's worth the effort.

Garry



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