Give the same project nine months and it will be completed in 18 months. It will also have low quality, mediocre performance, and serious design problems that persist for the next two versions.
Give the project only six months and it may, if you're lucky, reach some sort of satisfactory state in two years. You'll lose half your development team and 90% your customers. You'll wish for the remaining developers and customers to disappear too, but they won't
The way to achieve a software project with the right features, high quality, and on time is to to get experts to predict what you can do properly in the one year that you actually have.