exFAT external HDD changes 'Last modified' date by one month

I have a 1 TB exFAT external HDD mounted on my Ubuntu 20.04 system and am copying a large assortment of file (photos etc) to it.

After copying, the files seem to come across correctly. File contents, the 'Last Modified' date... everything seems in order.

However, when I unmount and re-mount the drive, I find that the 'Last Modified' date for every file and every directory has spontaneously increased by exactly one month (otherwise the day and timestamp remain correct, down to the second), in many cases moving the date into the future. This also appears to happen sometimes without unmounting and re-mounting, though I haven't found any rhyme or reason to when exactly.

After unmounting and re-mounting multiple times, the dates don't continue to increase by a month every time. They simply remain stuck one month ahead of the original correct time.

I'm at a complete loss to explain this. How on Earth could this be happening?

Solution 1:

This is a known bug in Ubuntu 20.04 "Focal Fossa"
https://bugs.launchpad.net/ubuntu/+source/ubuntu-meta/+bug/1872504

Installing exfat-fuse and exfat-utils fixes the behavior

$ sudo apt install exfat-fuse exfat-utils

see also this question & answer, very similar to yours: https://askubuntu.com/q/1276694
Here the user reported they had to specify the timezone in fstab as well (in addition to installing exfat-fuse and exfat-utils)