How do you plan when the date is fixed

Fix the date and vary the scope, explicitly and in writing, with the cut list agreed before work starts. The failure mode is holding both fixed, which does not change the outcome, only when everyone finds out.

Keep the buffer at the project level rather than per task, and make spending it visible. Per-task padding disappears into Parkinson's law; a shared buffer with a burn chart stays honest because everyone sees the same number.