Working back from a fixed date
The forward question is when work can finish. The harder one is when it has to start.
The latest start is a point of no return
Every date in a backward schedule is the last moment an activity can begin and still hit the deadline. Read as a plan, that is a schedule with no float anywhere — one bad morning and the date is gone.
Read as a warning, it is exactly what you want: the day after which the deadline is no longer achievable without changing something. Treat it as a countdown, not a target, and start earlier than it says.
When the answer is in the past
If the latest start has already gone, the deadline is unreachable with the plan as written. That is a finding rather than a failure, and the sooner it is stated the more options remain.
There are four honest responses and they never change: shorten the work, remove some of it, overlap what was sequential, or move the date. Discovering which one you need in month one is a completely different conversation from discovering it in month five.
Holidays break these calculations quietly
Scheduling backward across a shutdown or a run of public holidays behaves very differently from scheduling forward across the same period, because the compression lands somewhere else entirely.
Enter the real non-working days rather than approximating. A fortnight of shutdown between now and the deadline can move the latest start by three weeks.
The deadline is the last day worked
An easy off-by-one: a deadline of the 30th usually means work finishes on the 30th, not that work stops at the start of it. And if the deadline falls on a weekend, the real last working day is earlier.
Getting this wrong shifts an entire backward schedule by a day, which is invisible until it is expensive.
It moves the slack to the front, where you can see it
Planning forward from today, every task starts as early as it can and whatever spare time exists piles up at the end, just before the deadline. It looks comfortable, and it is the least useful place for it: the buffer is furthest from the work most likely to overrun, and it is the first thing anyone spends when an earlier task slips.
Planning backward from the deadline does the opposite. Every task starts as late as it can, and the spare time appears at the front as a gap before anything has to begin. That is uncomfortable to look at, and it is honest: it tells you how long you can wait before the plan stops being achievable, which is the number you actually wanted.
The plan it produces is deliberately fragile
Worth being explicit about the trade. A backward plan has every task at its latest possible start, so by construction nothing has any float at all — any slip anywhere moves the end date, because there is no absorption anywhere in the chain.
That makes it excellent for answering "when must this begin?" and bad as a schedule to run a project from. The usual practice is to use it to find the latest start, then deliberately begin earlier than that, treating the difference as a buffer you have chosen and placed rather than one that accumulated by accident. A plan executed exactly as the backward pass computed it will hit its deadline only if nothing goes wrong at all.