The ability to browse the dashboard by date, jump back a day, a month or even a year. The limit is the date of the earliest post of any of the blogs you follow.
The archive shows all the posts of all the blogs you follow to date regardless of when you followed them and when you joined Tumblr, allowing you to visit the posts you missed. Posts from blogs you have since unfollowed are not displayed.
A dashboard currently being browsed in the past works like the present dashboard. Continuous scrolling loads older posts, subsequently browsing into the previous month and so on. Posts show up to date information such as notes, featured tags and changes made to the post post-posting (yes, I said “post post-posting”), deleted and private[d] posts are not displayed.
In the dialogue window (pictured), the arrows (left and right) jump between years, clicking a month will display the days with the total posts, posts are ordered from 23:59 back to 00:00 and continuing into the previous day. Next to each month in the dialogue box is the total posts made (the posts of all blogs you currently follow posted within that month).
The option to go back in time is also present on blog dashboards, the inbox and the likes page. This feature is desperately needed for the likes page which is currently a waste of time.