This might seem a little odd, but is very useful when migrating backup sets from other software. For instance, I recently converted a huge set of tgz files to Bup, and specified the date and time for each, thereby recreating the backup history. It doesn't have much use outside of this scenario, but would allow migrated sets to play nicely with the attic prune command. I imagine it's also a fairly simple addition.

_Comparison note__: Bup and Obnam both allow this._