Day One offers four different views for your data. You can view your Timeline, Photos, Maps, and Calendar. Each one offers a different perspective on looking back on your memories.
The view options are along the bottom of the app. Each one is a tap away.
Using the Calendar view, you can view past entries or scroll to a date to create entries on past or future days. See here.
While scrolling through the Timeline, Photos, or Calendar view, you can tap on that icon a second time to quickly scroll to the current date.