How can all files on my drive suddenly have modified timestamps set to 1 hour newer than a week ago?

Question

I regularly back up most of my files using Robocopy using the mirror function.

I haven't noticed anything odd in the past days, since I most frequently backup to another SSD. But to my horror, when today I decided to backup to a relatively slow USB flash drive, it began copying everything!

I suddenly realized that it was because every single file on the USB flash drive was 1 hour older than all the files on my SSD.

The last time I backed up to this USB flash drive was about one week ago, so something must have happened in the last week to set the modified timestamp of every single file on my SSD one hour later than they were before.

What could possibly be responsible for this? I don't recall playing with the date-and-time settings in Windows, but…

And why should it change the timestamp? I could maybe understand Windows interpreting the timestamp differently, but then surely it should also interpret the files on the USB flash drive as having the same timestamp. So it seems that every file must have been individually touched?

What's weird is that when instead of Windows Explorer I tried using the command line WMIC DATAFILE [...] get LastModified on each file, then it didn't say that the files are 1 hour apart but instead only 2 seconds! But still with the files backed up last week being the ones having the lower timestamp value.

Answer 1

Two seconds difference is normal if the disk is formatted as NTFS but the USB is not. I would guess that the USB drive is formatted in the FAT or exFAT formats.

The Microsoft article File Times explains the problem:

The NTFS file system stores time values in UTC format, so they are not affected by changes in time zone or daylight saving time. The FAT file system stores time values based on the local time of the computer. For example, a file that is saved at 3:00pm PST in Washington is seen as 6:00pm EST in New York on an NTFS volume, but it is seen as 3:00pm EST in New York on a FAT volume.

This is why many backup programs compensate for this problem.

Robocopy for example has this parameter:

/dst    Compensates for one-hour DST time differences.

For best results, you should format the USB as NTFS, so that time calculations are the same for both and use Coordinated Universal Time timestamps that are translated to local time when displayed in Explorer.