If you have purchased Day One Premium and it is not showing up on all your devices, please follow these steps on the device where you purchased the Premium subscription:
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”
Day One > Settings > Advanced > Select "Restore Purchase"
If you do not see Premium on your other devices after these steps, please contact Day One support: dayoneapp.com/contact