The Voynich Manuscript has been carbon dated to somewhere between 1404 and 1438:
Using radiocarbon dating, a team led by Greg Hodgins in the UA’s department of physics has found the manuscript’s parchment pages date back to the early 15th century, making the book a century older than scholars had previously thought.
This rules out Leonardo da Vinci as the author, which was my personal favorite theory. da Vinci was born in 1452. It also rules out John Dee and Edward Kelly, who lived in the 16th century. It also of course rules out the possibility that Voynich forged the manuscript.