[tootie]

This is well-known and is the entire reason that agile exists. A lot of teams will write stories and run sprints and think they're doing it right. The actual definition is the ability to flex on scope and timing to meet changing requirements and priorities. Long-term estimation is just never going to be accurate, so setting a date and fixed scope is just automatically doomed.

The strategy I use its to scope out as much as you can up front. A list of high-level user stories. Give these a rough prioritization (MoSCoW works) and some rough estimates on each. Now estimate your velocity with a few possible team configurations. Also, assume your backlog will grow about 10% as you go when new stuff is uncovered.

Now if you need to schedule a launch or set a budget, set it deep into the non-mandatory features. If everything goes off the rails, you have cushion to avert failure. If everything goes ok, you will deliver a richer product. You'll also be able to track very accurately as you go how close you are to the plan week by week.