Fix backup calculation

(cherry picked from commit a4538635)

aleksis/core/health_checks.py

@@ -38,9 +38,9 @@ class BaseBackupHealthCheck(BaseHealthCheckBackend):
             self.add_error(_("The backup folder doesn't exist."))
             return
         if backups:
-            last_backup = backups[-1]
-            last_backup_time = dbbackup_utils.filename_to_date(last_backup)
-            time_gone_since_backup = last_backup_time - datetime.now()
+            last_backup = backups[:1]
+            last_backup_time = dbbackup_utils.filename_to_date(last_backup[0])
+            time_gone_since_backup = datetime.now() - last_backup_time
 
             # Check if backup is older than configured time
             if time_gone_since_backup.seconds > self.configured_seconds: