Leap Second does not store your entries. Your Photos app stores your entries locally on your device.
If you are changing phones or need to factory reset your device, migrate the storage to the app to ensure you don't lose your Journal entries.
Migrating the storage to the app is a Pro feature that works combined with enabling Leap Second Cloud storage.
Tap the three-button Menu available on the side of your Journal name to open your Edit settings.
Tap on Migrate Storage to App (Beta). It's the last option available.
It will open the Migrate Journal pop-up. Tap on Start migration.
You will now see a copied version of the journal named Migrated (Journal Name). Tap the three-button Menu again and Enable Cloud Storage.
Browse the calendar view, tap the "you have entries not backed up" banner, followed by Start Backup.