Here's an example: Let's say that you want your project to be finished on November 1st, so you can launch your product for Christmas season.

In the above example you know your finish date, so you start calculating the amount of time (in number of days) required to finish the project, you subtract the number of days from your finish date, and you get your start date. As long as the start date is not in the past, you should be OK.

I think this is a good idea, as you will be more pressured to finish the project on time.