By default, posts on your Tumblr blog are displayed in reverse chronological order, with the newest at the top. Only tag pages -- the list of all posts bearing a specific tag on your blog -- can be made to appear in chronological order, by appending the word "chrono" to their address.
For example, if you wanted the tag page for the "kittens" tag on your blog to display in chronological order, its address would turn from http://yourblog.tumblr.com/tagged/kittens/ to http://yourblog.tumblr.com/tagged/kittens/chrono. This works for other people's blogs as well as your own.
The easiest way to direct your visitors to a chronologically-ordered tag page is to create a special page redirecting to it. Links to pages can automatically appear on your blog theme without your needing to edit its code directly, saving you time and effort.
Click the person icon on the dashboard. If the blog for which you want to create a chronological tag page is your primary blog, skip the next step, since the menu entry you need is immediately visible.
Hover over the title of the blog until a person icon appears and then click it to unfold additional menu entries pertaining to that blog.
Click the Edit Appearance menu entry for the blog in question.
Click Edit Theme.
Click Add a Page. The link is at the very bottom of the theme options, so you may have to scroll down for it to become visible.
Select Redirect from the drop-down menu.
Enable the Show a Link to This Page option and type a title in the text field. Doing this automatically adds a link to the redirect page on your blog theme.
Type an address for the redirect page in the URL field. Your blog's address is automatically added, so all you need to include is the ending part.
For example, if you wanted the redirect page to be located at yourblog.tumblr.com/chronokittens, you would simply type chronokittens in the text field.
Type the full address for the tag page where you want the redirect to lead in the Redirect to field.
For example, if you wanted the redirect page to lead to http://yourblog.tumblr.com/tagged/kittens/chrono, that is exactly what you would type in the text field.
Click the arrow to return to the theme options. Add any other redirect pages in the same manner.
Click Exit to return to the appearance customization screen
Click the home icon to return to your dashboard.