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):
The Restore Purchases button must be done on the device where you originally subscribed. That will send the receipt info to the server and then to the other devices. The icon for iOS and Mac is slightly different. See below:
Day One > Settings > Advanced > Select "Restore Purchase"
Day One > Settings > Account Status > Select "Restore"
Go to Day One > Preferences > Support > Restore Purchases
Or on the upgrade screen there is a button for Restore Purchases
If you do not see Premium on your other devices after these steps, please contact Day One support.