Ability to backdate a backup

Hi,

I've got a series of manual backups which I'd like to "convert" to a duplicity backup in order to make use of it's advantages.
While I could just put the oldest backup in a location, create a duplicity backup and do this in the same location for all the subsequent backups, it will cause loosing the timestamp of when those backups where actually made.

Is there a way to backdate duplicity backups, either afterwards or when creating the backup? I haven't found suitable option for this in the documentation.