[mightybyte]

I don't think that's the way it works out in practice. The fact of the matter is that deadlines are missed all the time. In many cases, there is no such thing as 100% certainty that you'll hit a "deadline"--there are always circumstances outside your control (global pandemics anyone?). There's just some implicit confidence threshold or other assumptions lurking around that probably need to be communicated. Do you want three 9s of confidence? Five 9s? Those things are very different and the cost to actually achieve the latter can often be prohibitive. Everyone benefits if we make explicit our pre-conceived idea of precisely what "cannot exceed" means.

[staunton]

Going even further, deadlines are often a tool for signaling "the organization is trying really hard to achieve this fast". The shorter the deadline (as long as it's at least somewhat in theory plausible), the harder you're trying. Often most people involved (even those deciding on the date for the deadline) know from the very start that the deadline will almost certainly be missed.

I regularly see two kinds of deadlines. "Planning deadlines" describe an estimate when something will be done. "Signalling deadlines" signal priorities and motivation to employees or clients. Sometimes both exist in parallel for the same task and there is a subset of people who know both.