FreeFileSync Open Source File Synchronization
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FreeFileSync User Manual:
#Quick Start
Command Line
Scripting
Comparison Settings
Daylight Saving Time
Exclude Files
Expert Settings
External Applications
Macros
Performance
RealTimeSync
RTS: Run as Service
Schedule Batch Jobs
Synchronization Settings
(S)FTP Setup
Tips and Tricks
Variable Drive Letters
Versioning
Volume Shadow Copy
Daylight Saving Time (Windows)
Synchronization software often encounters ±1-hour file time shifts after a Daylight Saving Time (DST) change. This commonly occurs when comparing a FAT32- or exFAT-formatted volume (referred to as "FAT") against an NTFS volume, for example, when synchronizing a USB stick with a local disk. Files that were previously in sync are now shown as offset by one hour, even though they have not been modified.The reason for this behavior lies in the way NTFS and FAT 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 with the effect that file times that used to be the same show up as different after a DST switch or when the time zone has changed.
See Beating the Daylight Saving Time Bug for a detailed discussion on handling DST shifts in file times.
Solutions:
- In FreeFileSync's comparison settings you can enter one or more time shifts to ignore during comparison:
If you need to handle differences due to daylight saving time, enter a single one hour shift.
If the differences are caused by changing the time zone, enter one or more time shifts as needed.
Note
File times have to be equal or differ by exactly the time shift entered to be considered the same. Therefore, the time shift setting should not be confused with a time interval or tolerance.
- Alternatively, you can avoid the problem in the first place by only synchronizing from FAT to FAT or NTFS to NTFS file systems. Since most local disks are formatted with NTFS and USB memory sticks with FAT, this situation could be handled by formatting the USB stick with NTFS as well.