Only one subscription is needed for Premium on all platforms. If you have purchased Day One Premium (or Plus) and it is not showing up on all your devices, please follow these steps on the device where you purchased the Premium subscription (or Plus):
Day One > Settings > Advanced > Select "Restore Purchase"
Day One > Settings > Account Status > Select "Restore"
Follow the steps in this article to backup your data and reinstall Day One Mac:
Then, go to Day One > Preferences > Sync tab > Click “Subscribe for $3.99” > Click “Restore Purchase”
If you do not see Premium on your other devices after these steps, please contact Day One support.