I worked for several years on an extremely large and popular sports news/results website, meaning all our deadlines were absolutely set in stone by the outside world – sporting events start on specific dates. The Tour de France sure as hell ain't gonna postpone a few days until we've finished our team profile pages.

So we committed to a deadline, but not a single deliverable. Scope would shrink and grow as delivery of each component progressed. Well in advance, for each major sporting event in each country where we operated, we would scope out literally every single thing we'd like to deliver, and then start work on the components in priority order. In each planning session as the event approaches we'd appraise whether we still have time to realistically deliver the next priority thing, and if not we'd skip it and move to the next - those that got missed out could wait until the following year (when the major features already delivered would need to evolve, not be implemented from scratch).

I learnt a lot from working on sports. Having third party deadlines that no-one could bitch about made us an extremely effective team at planning, estimating, and ruthless scope reduction.

You can have a fixed scope with a flexible timeline, or a flexible scope with a fixed timeline. If you try to have both, you'll incur an additional cost, which most companies avoid. It's a well known law of project management and software development.