As I said in another comment software development is not at this point an engineering discipline. It is more of a craft. It is somewhat like the situation that bridge construction was in prior to the development of modern civil engineering. (And keep in mind that bridges still fail today albeit much less frequently.)

To get good estimates of how long projects will take you need to hire developers who have successfully completed multiple, very similar projects in the past. The less experience developers have at successfully building the specific type of system required, the worse their estimates of how much time and effort will be needed will be, and the higher the risk of outright failure.