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.
The 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.
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.
For a detailed discussion about this issue refer to: http://www.codeproject.com/KB/datetime/dstbugs.aspx
Solution:
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.
Note: In
order for FreeFileSync to start handling DST and timezone
differences, an initial full synchronization is required.
Subsequent syncs will never show a time difference again. 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.