What is a Time Fence? Definition & Manufacturing Examples

What is a Time Fence?

A time fence is a policy boundary within the planning horizon that defines different levels of authority and flexibility for making schedule changes. Inside the time fence, the schedule is frozen or semi-frozen, requiring higher levels of approval for any changes. Outside the time fence, the schedule is flexible and can be adjusted freely. Time fences protect near-term production stability while preserving planning agility for the future.

How Time Fences Work in Manufacturing

Most manufacturing organizations use two primary time fences:

Demand Time Fence (DTF): The near-term boundary inside which only firm customer orders drive production decisions. Forecasts are ignored inside the DTF because the schedule is committed to specific orders. Materials are allocated, machines are assigned, and operators are scheduled. Changes inside the DTF are expensive and disruptive — they waste committed materials, require schedule rework, and create overtime.

Planning Time Fence (PTF): The boundary beyond the DTF inside which the Master Production Schedule is firm. The system will not automatically change the MPS inside the PTF. Changes require planner or management approval. Between the DTF and PTF, the schedule uses a mix of firm orders and forecasts.

Beyond both fences, the planning system adjusts the schedule freely based on demand changes, forecast updates, and capacity conditions. This zone is far enough in the future that changes have minimal impact on current operations.

The position of time fences varies by industry and product. A make-to-stock consumer goods manufacturer might set the DTF at 1 week and the PTF at 4 weeks. A make-to-order aerospace manufacturer might set the DTF at 4 weeks and the PTF at 16 weeks due to long procurement lead times.

Time Fence Example

A manufacturer of industrial pumps sets the following time fences:

• Demand Time Fence: 2 weeks — Schedule is frozen. Only emergency changes with plant manager approval. All materials committed, shop floor schedules published.
• Planning Time Fence: 6 weeks — Schedule is firm. Changes require production planner approval. Materials ordered but not all received. Changes affect supplier deliveries.
• Beyond 6 weeks — Schedule is flexible. System adjusts automatically based on updated forecasts and new orders.

On Tuesday, sales requests adding a rush order due in 10 days (inside the DTF). The planner evaluates the impact: inserting the rush order requires bumping Job J-4510 by 2 days, which pushes it past its due date. The plant manager reviews the tradeoff — the rush order is worth $45,000 and the bumped customer has agreed to a 2-day extension. Approval is granted with documented justification.

Without the time fence policy, such changes would happen informally, creating schedule chaos. With the policy, every near-term change is deliberate, documented, and evaluated for impact.

Why Time Fences Matter for Production Scheduling

Time fences create the discipline that production schedules need to be effective. A schedule that changes constantly is worse than no schedule at all because it destroys credibility with shop floor personnel. When operators see the schedule change five times a day, they stop trusting it and revert to their own judgment about what to run next.

Resource Manager DB (RMDB) supports time fence policies, allowing planners to define frozen, firm, and flexible zones within the scheduling horizon. The system enforces approval requirements for changes within protected zones while allowing free optimization in the flexible zone. This balance of stability and agility is essential for reliable production execution.

Related Terms

• Scheduling Horizon — The overall time period that contains the time fences
• Master Production Schedule — The production plan that time fences protect from unauthorized changes
• What-If Analysis — The simulation capability used to evaluate the impact of changes within time fences before committing them

Frequently Asked Questions

Learn more in our complete manufacturing glossary or production scheduling guide.