What Is Planning Time Fence?
Also known as: time fence, demand time fence, DTF, PTF, planning fence
Definition
A planning time fence is a number of days from today inside which MRP will not automatically create or change planned supply. Changes in that window need planner approval, which keeps near-term production stable while the outer horizon remains flexible.
Planning Time Fence Explained
Close to today, the shop floor has already purchased material, scheduled machines and set up tooling, so large automated reschedules are disruptive. A time fence marks that stable window. Inside it, MRP either suppresses new planned orders or only issues action messages. Beyond it, MRP is free to create, delete and move planned orders.
Two fences are commonly distinguished. The demand time fence is the horizon in which forecast is ignored and only firm customer orders drive planning, so noisy forecasts do not trigger rebuilds. The planning time fence is the horizon in which the planning engine will not change supply on its own. In some systems the two are set separately per item, in others they are combined with a single frozen period.
Setup is per item or per item and site. SyteLine exposes planning fence settings on the item and item warehouse planning data, Infor LN uses time fences in its planning parameters, and SAP uses planning time fence and the planning strategy on the MRP views of the material master. The fence length is typically tied to cumulative lead time or the frozen period of the master schedule.
The tradeoff is responsiveness against stability. A fence that is too long makes the plant slow to react to real demand changes. A fence that is too short lets MRP churn orders the shop is already running.
Why It Matters
- It is the primary defence against schedule nervousness in the weeks the plant is actively building.
- It decides who is in control near term: the engine or the planner.
- A mismatch between fence length and real lead time causes either late supply or constant reschedules.
- Fences interact with master schedule freeze periods, so they need to be set together to avoid contradictory rules.
In Practice
Example: an assembly has a 20 day cumulative lead time and a planning time fence of 15 days. A new sales order arrives for delivery in 10 days. Inside the fence, MRP does not create a planned order for it. It raises an action message and the planner decides whether to expedite, split an existing job or tell sales the earliest date.
Frequently Asked Questions
What is the difference between demand and planning time fences?
The demand time fence controls which demand MRP uses, typically actual orders only, while the planning time fence controls whether MRP may change supply. They are often set to different lengths for different items.
How long should a planning time fence be?
A common starting point is somewhere between the item's own lead time and its cumulative lead time, adjusted for how stable demand is. Review it against actual expedite and reschedule frequency.
What happens to orders created inside the fence?
They have to be created or changed manually, usually guided by action messages. This keeps the plan stable but puts the work on the planner.
Related Terms
MRP (Material Requirements Planning)
MRP (Material Requirements Planning) is the ERP calculation that explodes demand through bills of materials, nets it against on-hand and on-order inventory, and offsets by lead time to produce timed purchase and work order suggestions.
Firm Planned Order
A firm planned order (FPO) is a planned order that a planner has frozen so MRP will not delete, resize or reschedule it. It still counts as supply in netting, and it stays a planned order rather than becoming a released production or purchase order.
MPS (Master Production Schedule)
An MPS (master production schedule) is the time-phased statement of which end items a plant will produce, in what quantities, and in which periods. It is the anchor demand input MRP explodes into component requirements.
Lead Time
Lead time is the elapsed time between initiating an order and having the material available for use. In ERP it is the offset MRP applies when back-scheduling a planned order from its due date to its release date.
Planned Order
A planned order is a system-generated supply proposal that MRP creates to cover a net requirement. It has a quantity and dates but is not yet a committed job, production order or purchase order, and MRP can delete or reschedule it on every run.
Go Deeper
MRP Nervousness Assessment
A 10-question diagnostic that scores how badly your MRP plan churns between runs - and pinpoints which levers will stabilize it.
SyteLine Planning Time Fence Configuration Guide
Configure SyteLine planning time fences with demand fence, planning fence, and frozen zone settings to stabilize production schedules and reduce MRP nervousness.
SyteLine Master Production Scheduling Best Practices
Master production scheduling in SyteLine with planning bill configuration, time fence management, rough-cut capacity checks, and schedule stabilization techniques.
SyteLine MRP Parameters Tuning Guide
Optimize SyteLine MRP parameters including order policies, lot sizing, safety stock, and lead times for accurate material requirements planning results.
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