The 18 months only starts ticking after the 1st exam is taken I believe - not when you start the groundschool.
How did I know that somebody I told not so long ago (As seen here
http://www.pprune.org/professional-p...ml#post4472115) would come back and bite on the a*se me for not being clear enough
As RudeNot2 said above it's when you pass the first exam and not when you enroll.