Include Day One in your iCloud backups on iOS
Day One data can be included in iCloud device backups on all iOS devices. This is automatically enabled when you install Day One and have iCloud Drive enabled and iCloud device backups. Follow these steps:
On your iPhone, iPad, or iPod touch using iOS 11 or later, go to Settings > [your name] > iCloud > Manage Storage > Backups > tap your current device.
Enable Day One under Choose Data to Back Up.
Go to go to Settings > [your name] > iCloud > iCloud Backup and tap Back Up Now.
This will include Day One data to iCloud Device backups. For more information about how iCloud Backups work, see: https://support.apple.com/en-us/HT204136.
Steps to restore from an iCloud backup
If you ever need to replace or restore your iOS device, you can restore from the latest iCloud Device backup and recover your Day One data. This method restores your entire device. Data will be restored from the most recent device backup stored in iCloud.
Please see the Apple guide on restoring from an iCloud backup here: https://support.apple.com/en-us/ht204184