Brilliant - thanks Greg, just was I was looking for. Right now I'm half way through re-syncing my backup copy from my main copy, having reformatted the backup disk as ext4. Once that completes and I'm happy that it's a complete and correct backup, I plan to reformat my main disk as ext4 as well and re-sync that from the backup (I do also have another backup in case of blunders!). Hopefully this will be an end to the timezone ambiguity.

Regarding the ext4 format, my main disk is an SSD, and I've read that it could be useful to turn off journaling to save wear on the memory. But given that the majority of the content on this disk is static (content only changes when I buy a new CD), is journaling going to be an issue? Are there still a lot of journaling disk writes when the content is only being read, not written? My backup disk (a conventional hard disk disk) seems to mount with -noatime by default, so I don't believe there should be any disk writes most of the time, but I don't know what extra is written by a journaling system.