You need to make allowance for some proper QA. Unit testing, BVA, integration testing, UAT, as well as system test. Usually takes at least as long as the development, although this is largely ignored by management which is why most software is a bug ridden, largely disappointing experience, requiring a large amount of support.
Thirty years in the business made me cynical? Ho yuss...
Edit: If I had a tenner for each time I'd seen the "Bugger - only a month to release, cut short the testing by 75% chaps - we just don't have the time money" scenario, I could probably retire. Thankfully - this hasn't extended to the two safety critical things I've been involved with over the years; one involving things that fly, the other involving things that could blow up but you really don't want them to. In those cases, they ran massively over schedule and budget instead, but at least they worked.