Not all Tumblr accounts show the timestamp for each post. However, while the owner of the account can remove the timestamp, you can still sort the posts on any Tumblr page if you know the correct URL to access. The default view for Tumblr posts shows the latest posts on top. By changing the URL, you can show the oldest posts on top and organize those posts by tag.
Access the Tumblr site with the post you want to view.
Check for a timestamp at the base of the post. If you don't see a timestamp, click the "Notes" link to view the comments. The timestamp usually displays on the Notes page. Sometimes you can also click the picture in the post to open a new page that features the post along with a timestamp.
Click one of the tags on the post you want to view, and then append "/chrono" (without the quote marks) to the end of the URL to view the posts in chronological order. This displays the post with the oldest timestamp first.