S3 backup question

Hey guys, Quick question:

I just setup an S3 account and configured Virtualmin to do daily incremental backups to it. My bucket name looks like this:

I-M_Backup-%m-%d-%Y

I've enabled strftime conversions and it's correctly creating the buckets on S3. However, I really only want to keep 30 days worth of backups, so I enabled the "delete backups after" and set it to 30 days. When I submit that it gives me an error that I'm not using a directory or filename that uses the date.

Is this something that's a bug, or something that's not supported by S3 backups? I would really hate to have to go in and manually delete my buckets every so often, 'cause I'll forget. :)