A new feature in macOS Sierra (10.12) enables users to save space locally by enabling "Optimize Mac Storage" in the iCloud settings. This has caused some older entries in Day One that are synced with iCloud to disappear from the app. Please try these steps to troubleshoot this issue.
Quit Day One Classic
Open System Preferences > Apple ID > uncheck the option for "Optimize Mac Storage".
On that same settings menu click Options next to iCloud Drive, make sure Day One Classic is checked in the list.
4. Re-open and Verify in Day One Classic that your older entries have returned