1999 is said by Roland Deschain to be the "Keystone Year in the Keystone World". This is the year that Stephen King gets hit by an oncoming van and is killed, never finishing The Dark Tower Series. There were clues to this, as "Nineteen" and "Ninety-Nine" were important numbers through out the series.
This is the year Roland and Jake go back to save Stephen King and Jake sacrifices himself in the process. Later Stephen King would repay the favor, leaving a note to Susannah Dean in Dandelo's bathroom warning her to look at Odd Lane sign (which was spelled Odd's Lane, to cover up the fact it was an anagram for Dandelo, whom Eddie Dean warned the Ka-tet of).