My answer is more complex than that of @Adam Morris, but with some differences. On refection I think the complexities are not needed and that @Adam Morris's answer is correct and sufficient. I'm leaving this answer (which I believe to be correct) in case it is useful to others.
man tmutil says that the command is:
tmutil delete [-d backup_mount_point -t timestamp ...] [path ...]
What is the mount point? There are two steps:
- With Finder look in the snapshot's volume on the Time Machine disk.
- List the mount point with the command
mount. This includes lines like this:
com.apple.TimeMachine.2020-12-08-145707.backup@/dev/disk10s2 on /Volumes/.timemachine/8A653E1B-09C0-44C1-9F6C-EF28F0F7A087/2020-12-08-145707.backup (apfs, local, read-only, journaled, nobrowse)
So I have tried the command:
sudo tmutil delete -d /Volumes/.timemachine/8A653E1B-09C0-44C1-9F6C-EF28F0F7A087/2020-12-08-145707.backup -t 2020-12-08-145707
The immediate response is:
Total deleted: Zero KB and the folder on the TM drive has gone.
But has the backup really gone? I am not sure!!
So my suggestion is that the command should be like this:
sudo tmutil delete -d /Volumes/.timemachine/<UUID>/<date-time>.backup -t <date-time>
where the UUID and date-time are from running the