The use case I'm thinking about is with a room of shared Macs in an educational setting. It might be useful to keep a post-setup snapshot to use as a restore point for a number of weeks/months.
The docs say that Time Machine will delete snapshots after a day (which you can confirm is happening with "tmutil listlocalsnapshots /").
Is there any way a snapshot created manually (with "tmutil snapshot") can be preserved for an arbitrary amount of time?
