Sadly far too often managers and decisionmakers think timeline and deadlines is a topic for haggle, not discussion. They treat it same as haggling on a bazaar.
If I'm asked for an estimate to do X and I say I can deliver in three weeks, and my boss says customer needs it for golive in three days, I'll try to find some way of making that work. Perhaps they can live without a full solution for the first few weeks, instead requiring only a subset of the requirements in that period. Or, if I insist it just cannot be done, I'll tell the boss who'll try to push back golive if it's important enough.
There needs to be respect for each other and the project though.
Of course on any project I might be able to deliver the same in less time, but then at the expense of something else. That might be acceptable if the boss thinks he can manage the other clients which work gets delayed.
If I'm asked for an estimate to do X and I say I can deliver in three weeks, and my boss says customer needs it for golive in three days, I'll try to find some way of making that work. Perhaps they can live without a full solution for the first few weeks, instead requiring only a subset of the requirements in that period. Or, if I insist it just cannot be done, I'll tell the boss who'll try to push back golive if it's important enough.
There needs to be respect for each other and the project though.