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authorDaniel Wilhelm <daniel@wili.li>2014-04-18 17:08:42 +0200
committerDaniel Wilhelm <daniel@wili.li>2014-04-18 17:08:42 +0200
commitc32707148292d104c66276b43796d6057c8c7a5d (patch)
treebb83513f4aff24153e21a4ec92e34e4c27651b1f /BUILD/Help/html/advanced/DaylightSavingTime.html
parent3.9 (diff)
downloadFreeFileSync-c32707148292d104c66276b43796d6057c8c7a5d.tar.gz
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3.10
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+<BODY LANG="en-US" DIR="LTR">
<H2 CLASS="western" STYLE="font-style: normal"><FONT FACE="Tahoma, sans-serif">Daylight
-saving time</FONT></H2>
+saving time <SPAN STYLE="font-weight: normal">(Windows)</SPAN></FONT></H2>
<P STYLE="margin-bottom: 0cm"><BR>
</P>
+<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">A
+common problem synchronization software has to deal with are +-1 hour
+file time shifts after a Daylight Saving Time (DST) switch has
+occurred. This can be observed for example when a FAT-formatted
+volume is compared against an NTFS volume as frequently happening
+with USB memory sticks. Files that previously appeared to be in sync
+are now shown with an one hour modification time offset, although
+they have not been modified by the user or by other means.</FONT></P>
<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">The
-Windows operating system exhibits quite a unique behavior when
-handling timestamps on NTFS file systems. This leads to a well-known
-issue that all file synchronization software has to handle:</FONT></P>
-<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">During
-daylight saving time switching, (local) file times are shifted by one
-hour on NTFS file systems, while NO shift occurs on FAT/FAT32 drives
-(as used by most USB sticks)! Thus when synchronizing an NTFS against
-a FAT32 volume file times become asynchronous twice a year!</FONT></P>
+reason for this strange behavior lies in the way NTFS and FAT drives
+store file times: NTFS stores time in UTC format, while FAT uses
+local time.</FONT></P>
+<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">When
+times stored in these two different formats are compared, one format
+has to be converted into the other first. In both cases Windows uses
+the current DST status as well as current time zone information for
+its calculations. Consequently the result of this comparison is
+dependent from current system settings and in particular file times
+that used to be the same can show up as different after a DST switch.</FONT></P>
+<P STYLE="margin-bottom: 0cm"><BR>
+</P>
<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">For a
-comprehensive discussion about this issue refer
-to:<BR><A HREF="http://www.codeproject.com/KB/datetime/dstbugs.aspx">http://www.codeproject.com/KB/datetime/dstbugs.aspx</A></FONT></P>
+detailed discussion about this issue refer to:
+<A HREF="http://www.codeproject.com/KB/datetime/dstbugs.aspx">http://www.codeproject.com/KB/datetime/dstbugs.aspx</A></FONT></P>
<P STYLE="margin-bottom: 0cm"><BR>
</P>
<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif"><B>Solution:</B></FONT></P>
-<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">In
-order to avoid re-copying files that have a modification time
-difference of +-1h FreeFileSync offers a global option: <I>Menu -&gt;
-Advanced -&gt; Global settings: Ignore 1-hour file time difference</I>.</FONT></P>
-<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">If
-&quot;<I>Ignore 1-hour file time difference</I>&quot; is checked, all
-files with a +-1h difference are treated as having the same time.
-This also handles different file time precisions (NTFS: 100ns,
-FAT/FAT32: 2 seconds) by allowing a 2 second tolerance .</FONT></P>
-<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">Files
-that have a deviation of less than one hour are categorized as
-conflicts. The reasoning is that these files surely have been
-modified on at least one side since last synchronization. But during
-one of the two daylight saving time switches per year the older file
-is shown pretending a newer date which can lead to data loss (when
-using the rule to overwrite older with newer files).</FONT></P>
-<P STYLE="margin-left: 1.46cm; margin-bottom: 0cm"><SPAN ID="Rahmen1" DIR="LTR" STYLE="float: left; width: 80%; height: 0.14cm; border: 1px solid #000080; padding: 0.05cm; background: #ccccff">
- <P ALIGN=LEFT STYLE="margin-left: 0.79cm; margin-right: 0.98cm; margin-bottom: 0cm">
- <FONT FACE="Tahoma, sans-serif"><B>Note:</B><BR>If synchronization
- takes place between NTFS &harr; NTFS or FAT32 &harr; FAT32 this
- checkbox provides no advantage and can be left unchecked.</FONT></P>
-</SPAN><BR CLEAR=LEFT><BR>
+<P STYLE="margin-bottom: 0cm"><FONT FACE="Tahoma, sans-serif">Luckily
+FreeFileSync users need not to worry about this issue. Each file on a
+FAT volume automatically gets additional meta data encoded in its
+creation date that enables a correct file time calculation. This not
+only solves all DST issues but also time shifts that occur due to
+travel between different time zones.</FONT></P>
+<P STYLE="margin-bottom: 0cm"><BR>
</P>
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