DST (Daylight Saving Time)
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Daylight saving time (DST), or summer time in British English, is the convention of advancing clocks so that evenings have more daylight and mornings have less. Typically clocks are adjusted forward one hour in late winter or early spring and are adjusted backward in autumn. Details vary by location and change occasionally; see When it starts and ends below.
DST affects businesses, traffic accident rates, and energy usage patterns, and it complicates computer-based systems.

The Daylight Saving Time Problem when comparing files
If comparing files from different file systems (computer to computer, computer to external disk, computer to other media), you may notice timestamps that are exactly one hour different, particularly when Daylight Savings Time (DST) starts or ends.
Under NTFS file system (and some other file systems) file timestamps are stored in "file time" and then translated to local "system time" (the date/time you see on Windows Explorer or Comparator Fast) by a formula based on the timezone, and whether or not DST is in effect.


In locations where DST is used, twice per year when DST starts or stops, the formula changes, but the same formula for calculation remains. Therefore, if before the time change you made note of a timestamp on a file on your server and you looked at it again today, you'd see that the timestamp appears to have jumped forward or backward one hour.

The FAT file system, used by Windows 95/98/Me and in moderns Windows versions (applied to external media devices, like Pen Drives) works different. Timestamps are stored as is and aren't affected by the timezone or DST. This way, the reported timestamp of a file in FAT file system, does not "jump" when DST changes.

If you have all NTFS-based file system, or all FAT file systems, you probably wouldn't notice any of this. But when a Windows 95/98/Me PC is attached to a Windows computer with NTFS file system and you move/compare files between them, then it's a significant problem when DST is active. And a 2 second difference on reported file's date/time appears when comparing files in NTFS file system against FAT file system (this is handled using threshold on Scan Parameters).

Comparator Fast have the "Automatic ignore DST difference" option, just need to enable (check) it, and if comparing two files with same file size, but Date/Time is exactly one hour different, the file is considered updated due to Daylight Saving Time.

See Also: File Time Threshold, Automatically ignore DST difference.