I'm a big fan of Elite:Dangerous, but I think you are way too optimistic with that date - middle to end of 2015 seems much more realistic:
When they say "from release" they likely mean "when it is available for the general public to buy". Currently the release schedule looks like this:
Alpha release - December 2013
Beta Premium release
Beta Standard release
An extended Gamma release (available to all Kickstarter backers but no-one else)
The first Beta is supposedly January 2014 (according to their original Kickstarter dates), but given they've subsequently said they'll take as long as they need (especially with the Alpha), I think February or March 2014 is more likely.
Then second Beta is supposed to be a month after that, so say March or April 2014.
Then the "extended Gamma" is supposed to be a month after that, so say April or May 2014. They've hinted this will be for an "extended" period. And as this makes the game available to 25,000 backers from the Kickstarter, I think we can expect them to spend much time tweaking the game to work well with such large numbers (including the cheating/hacking we must expect in a multiplayer game). It's optimistic to say this will only take 2 months, but I think it might be as long as 6 months - they want it to be as good as possible before they make it available for game reviewers (and the general public).
So the public release could be anywhere from June to December 2014!!
And that means that the Seamless Planetary Landings expansion won't be released until AT LEAST June to December 2015.