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TNCS-0021 - Handling Daylight Savings Time Issues

October 27, 2014


If you’re in a location that observes Daylight Savings/Standard time changes, and you’ve targeted FAT-formatted volumes in your ChronoSync sync documents, you may notice that ChronoSync has triggered everything to be synced when the time changes.


The best way to deal with Daylight Savings is to use a filesystem that uses UTC to store time. HFS+ and NTFS both do this already, but FAT32/16 (which many external drives and NAS devices use) does not. If the drive is being used on both Macs and PCs, consider formatting the drive as exFAT (from a PC). ExFAT is compatible with both Macs and PCs, and uses UTC to store time, eliminating this issue entirely. It also does not have the 4GB file limitation that FAT32/16 filesystems have.

Another technique you can use if you know the two targets are already in sync is to just perform a baseline scan after a DST transition. This can be done via the menubar by going to Actions -> Synchronize -> Baseline Scan. This tells ChronoSync to record the modification dates (and other attributes) of all files in the sync and consider them current. After doing this,ChronoSync will just look for relative changes (relative to the last known state of the files).

Finally, if you’re unable to reformat the drive to use a more current filesystem, and you know there are items that need to be synced, then unfortunately the solution is to sync. It will happen twice a year but nothing can be done because of the incapability between formats of HDs. If the different formats kept track of time the same way this would not happen. So it is not a ChronoSync issue but a format issue.


ChronoSync keeps track of all file and folder timestamps. If a filesystem does not report accurate information about a file (e.g. it reports the modification date relative to another time zone), it doesn't mater. We record the state of the file as the filesystem reports it. Thus if the time-stamp is off by an hour, we record it as being off. If it changes relative to itself, we'll detect it. Add to this additional triggers such as file size and you can see it is very difficult for a legitimate file change to go undetected by ChronoSync.

The only significant problem is that some PCs will suddenly report time-stamps that are off by an hour when a DST transition occurs. In such instances, ALL files may appear to have changed. The "Allowed time difference" feature in the Sync Triggers section of the Options Panel can be used to combat this problem by requiring that file modification dates differ by more than an hour. Since you can still trigger a sync by file size, it is very likely that files that have legitimately changed within the 1-hour window would still be detected and synchronized. Even though this can be done it’s recommended you sync everything so that all files are the same.


Oct-27-2014 - Created from Internal Support Notes.