Either
/mnt/backups/serv1-%Y-%m-%d
or
/mnt/backups/serv1-%Y-%m-%d.tar
depending on if want to save all domains to a folder or to a single file.
First example above will create new date based folder each time and put all domain’s backups in it as separate files.
Example two will create a date based file in which all domains will be zipped, tar’d, bz’d. Or which ever extension you use.