You can adjust the date to anytime past, present, or future in Day One. This allows you to retroactively create past entries from memory or using photos.
While editing an entry, tap the paper clip icon 📎, then tap the Date.
Or tap the date at the top of the entry or tap the map at the bottom to open the metadata view. Then tap the Date field to edit.
In a new entry, click the clock icon at the bottom of the entry.