It's a flawed thought, since it's only the date they complete the acquisition on paper i.e. to signal to everyone that it's permanent. This does not forbid you from working with the company for integration purposes, or why would you have such a long time span between announcing you agreed to buy somebody and actually 'doing it'?

In reality you would do a lot of work with the company you're attempting to buy. If for example Skype promised they could change the network to enable wiretapping yet they are unable to, then MSFT could cancel the deal. Or indeed for any of the reasons outlined in the original press release about 'forward-looking statements'.