Yearly retention

Hi,
we are using btrbk to send snapshots of backuppc to remote location for 1-2 years.
We have this policy
```
snapshot_preserve_min 1d
snapshot_preserve 5d

target_preserve_min 1d
target_preserve 5d 5w 5m 1y
```

Recently, i have noticed, that yearly snapshot is from 1.1.2017 .
Is there a way to configure it in a way, that it preserves oldest snapshot, until there is new, which is one year old? Then to drop the old one and keep next until there is another one to replace it again?

Thanks,
Libor