Fix snapshot retention times

The current period should not count, so e.g. monthly=3 results in the 3
_previous_ months snapshots being kept (plus the snapshot for the current
month.

zfs_smart_snapshot/main.py

@@ -13,7 +13,7 @@ def keep_time(now, snapshots, convert, delta, count):
     keep = set()
     if count < 1:
         return keep
-    start = convert(now) - delta*(count-1)
+    start = convert(now) - delta*count
     for snapshot in snapshots:
         snapdate = datetime.strptime(snapshot, 'zfs-smart-snap-%Y-%m-%d-%H%M').replace(tzinfo=timezone.utc)
         if snapdate >= start: