"you can trivially compare using normal bash string comparison functions" – Only if the offset (e.g.
+0100) is the same. The actual date is most likely stored as seconds since the Epoch or so, but btrfs subvolume show shows it in the timezone used by the OS. If e.g. DST starts or ends during the (some time passes) phase then old_date and new_date will not be trivially comparable and the result of your test may be wrong. To get really comparable results one shall use something like TZ=foo+0 btrfs subvolume show / each time.