Comment by kubb

You're saying it would be convenient for you to know the future. It would also be convenient for me. That said, if you haven't done very similar work in the past, it's very unlikely you'll know exactly how much time it will take.

In practice developers have to "handle" the people requesting hard deadlines. Introduce padding into the estimate to account for the unexpected. Be very specific about milestones to avoid expectation of the impossible. Communicate missed milestones proactively, and there will be missed milestones. You're given a date to feel safe. And sometimes you'll cause unnecessary crunch in order for a deadline you fought for to be met. Other times, you'll need to negotiate what to drop.

But an accurate breakdown of a project amounts to executing that project. Everything else is approximation and prone to error.