A common problem synchronization software has to handle 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 is the case when synchronizing a local disk against a USB memory stick. 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 the operating system.
The reason for this behavior lies in the way NTFS and FAT drives store file times: NTFS stores time in UTC format, while FAT uses local time.
When times of these two different formats are compared, one format has to be converted into the other first. In either way Windows uses the current DST status as well as the current time zone 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 or when the time zone is changed.
For a detailed discussion about this issue refer to: http://www.codeproject.com/KB/datetime/dstbugs.aspx
Solution:
FreeFileSync automatically handles this problem by adding the missing time information. 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.
Note In
order for FreeFileSync to start handling DST and timezone
differences, an initial full synchronization is required.
Subsequent syncs will then never show a time difference again for unchanged files.
If a FAT volume is scanned the
first time by FreeFileSync this will take longer than usual since
additional meta data is written for each file.