Management: We've got a great new project that has all the features that our users have been asking for. We need it done as fast possible, it's the number one priority. How fast can we get it done?
Engineers: For all of it? At least a year, probably more. Even a subset of those features will take six months, minimum.
Management: That's too long. You can have all the resources you want, we need to have it done in 90 days.
Engineers: That cannot be done. No amount of effort or engineers will get it done in 90 days.
Tell them to rank the features by importance. Go down the list with individual ETA estimates and they can deduce the final ETA. For bonus points provide error bars for each item so they can see how it adds up...
Engineers: For all of it? At least a year, probably more. Even a subset of those features will take six months, minimum.
Management: That's too long. You can have all the resources you want, we need to have it done in 90 days.
Engineers: That cannot be done. No amount of effort or engineers will get it done in 90 days.
Management: It will be done in 90 days.
180 days later
Management: Why are so many of our projects late?