To find the oldest book in your personal Mount TBR, sort the TBR books "in your hands" by registration date ascending. The first one listed should be the book that is your oldest TBR.
My oldest TBR is "At the Still Point", a memoir by Carol Buckley. It is from when I first joined bookcrossing, 2004. I am going to display the link to this book along with the title and author. When I have read this, I will display the release.
If you find that you truly do not want to read this book, then mark it available and find what your oldest TBR truly is.
After you have read your oldest TBR, you can then read the new oldest TBR. This challenge is for the year 2016, but people can start it now if they want.