You can keep Day One synced across any number of iOS mobile devices and/or Mac computers & laptops. Note that separate purchases are required for the mobile app and the Mac app, if you wish to use both.
Day One currently supports the following sync options:
iCloud sync is enabled by default on both platforms, and will work automatically if you're logged into the same iCloud account on all your devices, and if iCloud Documents & Data sync is enabled in device settings (if you are not using iCloud Drive) or if iCloud Drive is enabled in device settings.
You can use Dropbox by going to Settings > Sync (iOS) or Preferences > Sync (Mac). You must have the Dropbox app installed on your mobile device(s) and/or your computer(s) running the Day One app. If you don't already have one, create a Dropbox account.
Day One Sync was added to the iOS app in v1.17, and to the Mac app in v1.10. To enable Day One Sync, go to Settings > Sync (iOS) or Preferences > Sync (Mac) and toggle on Day One Sync, then follow any additional on-screen setup instructions. After enabling sync on each of your devices, your data will be uploaded for syncing. The exact amount of time to initially upload your data varies based on the size of your journal and the speed of your device and Internet connection. If you get any new devices in the future, simply enable Day One Sync, then your data will be downloaded to these devices. Learn more about Day One Sync.
Troubleshooting Sync Issues
Please see the following articles if you're experiencing issues with syncing your Day One entries: