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, as of January 1, 2017, is "Twice Told Tales" by Nathanial Hawthorne.
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, please post the name of the book and link to this book on this forum thread. Then you can then read your new oldest TBR book. This challenge is for the year 2018. Please join anytime during the year.