Welcome to the time machine that sends you back through tumblr.
A few notes about how this works:
You'll need to go to your Settings -> Dashboard and make sure Endless Scrolling is disabled, and Best Stuff First is disabled.
In my experience, you can only go to older posts when browsing. I don't know why. I'm pretty sure it's a tumblr thing
Tumblr seems to prioritize posts from people that have posted recently. Inactive blogs don't appear for me unless I unfollow the active blogs.
The Time Machine picks the smallest post number for the date you give it. Ex, selecting Feb 1st will show you posts from Jan 31st and earlier.
I don't have any post data from before Aug 2008, so you can't go there.
THE POSTS YOU ARE ABOUT TO SEE ARE PRODUCTS OF THEIR TIME. EACH POST YOU SEE ON THIS DASHBOARD IS BUT A FOOTPRINT IN THE PATH THAT THE USER HAS TAKEN TO REACH TODAY. THE FURTHER BACK IN TIME YOU GO, THE LONGER THE DISTANCE BETWEEN THE USER YOU SEE AND THE USER YOU KNOW. DO NOT ENTER THIS TIME MACHINE UNTIL YOU ARE READY TO HOLD SOME COMPASSION FOR THE GROWTH OF THE USERS THEREIN.