3.1 Schedule Constraints and Their Impact on Dates and Float
Key Takeaways
Relationships model dependencies, calendars model working time, and date constraints impose boundaries or fixed dates; these mechanisms should not be conflated.
Flexible constraints generally set one-sided date bounds, while mandatory constraints can override calculated dates and obscure network behavior.
A constraint should have a documented source, purpose, responsible owner, and review disposition.
Negative or positive float may result from comparing network dates with constrained targets, subject to calendars and software settings.
Use logic, milestones, resource analysis, or explicit activities when they better represent the underlying requirement.
3.1 Schedule Constraints and Their Impact on Dates and Float
A CPM network calculates dates from activities, durations, calendars, and relationships. A date constraint adds a boundary or fixed date. Constraints can represent real requirements, but they can also hide missing scope or logic. Review both the source and the mathematical effect.
Different scheduling mechanisms
- Relationship: represents dependency between activity endpoints.
- Calendar: defines working and nonworking time.
- Milestone: represents a zero-duration event or commitment.
- Resource rule: limits work according to available capacity.
- Date constraint: restricts an early or late date, or fixes a date depending on type and software.
If one crew must move from Area A to Area B, explicit crew-flow logic or resource-constrained analysis may be clearer than a start-no-earlier-than date. If a regulator permits work only after a stated date, an external milestone and relationship may provide better traceability than an unexplained constraint.
Flexible constraints
A start-no-earlier-than constraint generally sets a lower bound on early start: logic can push the activity later but not earlier. A finish-no-later-than constraint generally sets an upper bound used in the backward pass and can generate negative float if the network finishes later. Exact names and calculation behavior vary by software, so confirm configuration.
Flexible constraints can be legitimate for access dates, contractual windows, permits, or other requirements. Document the governing source and do not use them to conceal an activity that should be modeled.
Mandatory constraints
Must-start-on or must-finish-on constraints commonly force a date and can override or disconnect normal early/late calculations in ways that vary by product. They may create misleading float or prevent upstream delay from visibly moving the constrained activity. Use them only when required and understood, and explain the resulting calculation behavior.
A contractual milestone does not automatically require a mandatory constraint. Many schedules model the contractual date as a finish-no-later-than target or deadline so that the forecast remains visible and negative float quantifies the gap. Follow the contract, specification, and selected software methodology.
Float effects
Suppose unconstrained completion is 15 November and a required finish is 31 October, stated as eleven working days earlier under the applicable calendar. A late-date constraint may create negative eleven days of float on the driving path. That result says the current plan misses the modeled target by eleven working days. It does not identify fault or authorize a time extension.
An as-late-as-possible setting schedules an activity toward its late dates under the model. It can consume operational flexibility and increase exposure to disruption, but its detailed float behavior depends on software and network configuration. Avoid broad claims that every ALAP use automatically creates the project critical path.
Constraint audit
For every material constraint, record:
- activity or milestone;
- constraint type and date;
- source and authority;
- business or technical purpose;
- calculation and float effect;
- whether logic or an explicit activity would be clearer; and
- approval or disposition.
Revisit constraints during updates. A permit date may become an actual milestone, an access assumption may change, or a once-valid management target may be superseded. Preserve baseline history while keeping the current forecast technically honest.
The best model exposes why work cannot occur earlier or later. Use constraints deliberately, not as a substitute for planning or as a cosmetic way to force a desired bar-chart date.
Applied review: replace hidden dates with visible causes
For each constrained activity, ask what real event prevents earlier or later work. If the cause is an approval, delivery, outage, access release, or permit, consider modeling that event as a milestone with the work that produces it. This usually gives the update a responsible owner, status evidence, and a path that can move when conditions change.
Where a date constraint remains appropriate, run the schedule with and without it in a controlled copy. Compare milestone dates, float, longest path, and any broken driving relationships. Document the difference in the schedule basis or update narrative. This test shows users which dates come from execution logic and which come from an imposed requirement, improving forecast interpretation and recovery planning. Recheck the constraint after approved change, actual achievement, or a revised external commitment. Remove obsolete constraints through controlled change so they do not continue to influence later forecasts.
How does a must-start-on constraint generally differ from a start-no-earlier-than constraint?
Must-start-on fixes the date under the software’s rule, while start-no-earlier-than sets a lower bound and allows logic to drive later.
They are identical in every scheduling system.
Start-no-earlier-than deletes all predecessors.
Must-start-on is a resource-leveling rule.
What is the main concern with applying ALAP to procurement work?
It guarantees positive free float.
It can defer work toward late dates and reduce operational buffer, increasing exposure if supply or approval slips.
It eliminates the need for delivery milestones.
It automatically accelerates the project.
A finish-no-later-than target is eleven working days earlier than calculated completion. What is the narrow mathematical result?
Eleven days of free float are created.
The baseline is automatically revised.
The controlling path may show negative eleven working days of float under the stated calendar.
The contractor is automatically entitled to eleven days.
Sections you finish are checked off in the contents.