Backward Planning vs. Forward Planning for a Deadline
Backward planning starts from the fixed final deadline and works backward to determine when each phase must begin and end — well suited to a hard, immovable deadline. Forward planning starts from today and estimates forward based on how long each phase will realistically take — better suited when the deadline itself is somewhat flexible or not yet firmly set.
Both are legitimate planning approaches, and the better choice depends mainly on whether the deadline itself is fixed or still somewhat negotiable.
Backward planning
Starting from a hard, fixed deadline and working backward identifies exactly when each phase needs to start and finish for the final date to be met — this approach is most useful when the deadline genuinely can't move, since it directly surfaces whether the available time is actually sufficient for the planned work.
Forward planning
Starting from today and estimating how long each phase will realistically take, then summing those estimates to project a natural completion date, is more useful when the deadline is still flexible — this approach produces an honest estimate of when work will actually be done, which can then inform what deadline is realistic to commit to in the first place.
Using both together
A common, effective approach is starting with forward planning to get an honest estimate of natural completion time, then comparing that against the actual or desired deadline — if the forward estimate lands after the deadline, that gap needs to be addressed (more resources, reduced scope, or a genuinely later deadline) rather than simply hoping backward-planned milestones will somehow compress the real work.