It's what we call a longstop date, that if the property isn't ready or other conditions (eg at least a certain amount of the block sold) by x date then you can withdraw from the sale and get your deposit back in full.
Speak to your solicitor as to how the deposit is to be held (there are different ways eg as agent and stakeholder) to protect your deposit should the worst happen so your deposit is immediately available for return (see if you can get interest on it too). Deposits are often handed over to the developer after exchange so make sure it is safe eg stays with the developer's solciitors on their deposit account.
Have you got a mortgage offer as yet? If not then I'd look into this and check with your lender as it is getting much harder to get a decent mortgage on new build properties, especially so at the moment.
I'd also ask for the contract to be conditional upon having an appropriate mortgage offer (ie that it is suitable for you, not any old mortgage offer).
Also give NHBC a ring and ask them if they can explain the warrantly to you and what would need to be done (or, more importantly, not done - including communal areas and facilitie, entrances etc...) in order for them to sign the property off so you have idea of what you may be contracting yourself to move into!
http://www.nhbc.co.uk/