Two different dates
A deadline is a date chosen by people: a contract clause, a launch event, a promise to a customer. It is fixed by agreement and it does not care what happens inside your project. The end of your critical path is a different animal: it is a forecast, computed from your tasks, dependencies and working days, and it says the earliest your project can actually finish. (New to the concept? Start with what the critical path is and come back.)
One date is a commitment. The other is a measurement. Treating them as the same thing is how projects end up "suddenly" late in the final week.
The gap is your real status
Put the two dates side by side and subtract. Deadline September 30, critical path currently ending September 22: you have a buffer of 6 working days. That number is the most honest one-line status your project has:
- Positive buffer. The plan holds, and you know exactly how much bad news it can absorb.
- Zero buffer. Every critical task is now a single point of failure; one slipped day is a slipped project.
- Negative buffer. The plan already misses the date. Nothing dramatic has happened yet on the calendar, but the schedule is telling you the promise is broken, months early, while there is still time to act.
Why watching the deadline alone fails
The deadline never moves, so it never tells you anything. Weeks pass, tasks slip one by one, every status meeting says "mostly on track", and the date on the wall looks exactly as far away as it did last month. Then the last dependencies collide, and the project is late all at once, in its final stretch, when no options remain.
The forecast is the opposite: it converts every slip into information on the day it happens. A two-day slip on a critical task moves the forecast two days, immediately. A slip on a task with float moves nothing, and correctly so. Watch the forecast and you learn about September's problem in June. So the answer to the question in the title: watch the critical path, because the deadline cannot warn you about anything.
When the forecast crosses the deadline
Sooner or later it happens: the critical path ends after the promised date. There are exactly three honest levers, and all of them work on the critical chain only:
- Compress critical tasks. Add a person, split a task so parts run in parallel, or overlap steps that were needlessly sequential. Shortening non-critical work changes nothing, however satisfying it feels.
- Cut scope on the critical chain. Move a critical deliverable to version two and the path shortens today.
- Move the date, early and openly. A renegotiated deadline in June costs a conversation. The same conversation in September costs trust.
A 10-minute weekly routine
- Update actuals, then look at one number: the buffer between forecast and deadline.
- Watch the trend. A shrinking buffer is drift you can still correct cheaply.
- Glance at near-critical chains (float under 2-3 days): today's safe task is next week's critical one.
- Re-check after every scope addition. New work lands on some chain; find out which one before saying yes.