0 (zero) toggles between alphabetical and chronological sorting. This was added sometime in the 1.3.x development (in case you still use vdr 1.2).
I tried this, but it seems to only sort some recordings chronologically, not all recordings the way I expected.
Another "mystery" : when I erase a recording, vdr removes the *.rec directory and the files it contained, but leaves alone the main directory, though the permissions seem to be OK.
I recently changed the directory where vdr stores the files. May this change be the reason for those issues ?
Damien