If you have to finish on time

Value is not created until you ship your product, everything before amounts to costs. Getting stuff out the door is what counts. Fixing a release date may be an excellent motivator to make you ship. But, humans are terrible estimators, and we are overly optimistic when estimating the time it takes to finish a project. We feel we can do anything, whenever we need to. Which makes us pile requirements on to projects, and set lofty goals.

There is no way for you to set a sensible release date without knowing what it takes to ship. Instead of starting out by estimating the release date, start by doing a little bit of work, and then estimating.

A fixed release date can force you to ship. But, you have to maintain flexibility throughout the development process, and not fix the scope long before the release date. Be prepared to dynamically prioritise what goes into the next release, and shed requirements, when the deadline approaches.

In essence: If you have to finish on time, and you lock yourself down with a deadline, you have to maintain the flexibility to reduce scope as you along.