RFE: --delete-older

[bronger]

Use case:

Continual bi-directional synchronisation between two computers, while both systems may be updated at any time, so we have to handle some conflicts gracefully. This workflow is supported by the --update option but this covers only part of the problem. The other part are files newly created on the destination. They would be deleted with the --delete option, however, files created on the destination after the last synchronisation to the source are erroneously deleted, too.

I suggest a new option --delete-older=y-m-dTh:m which only deletes files on the destination side which are older than the given timestamp. Alternatively, a local file could be provided the timestamp of which is used. It would be orthogonal to the other --del… options.