JIT Manufacturing Reorder Window
Reverse-calc the latest purchase-order release day based on supplier lead time.
Written and maintained by Paul Clark, Redmoon Software · Rules last verified
These calculators are for informational purposes only and do not constitute legal, financial, or professional advice.
How the JIT Manufacturing Reorder Window works
The JIT Manufacturing Reorder Window calculator works backward from the date material is needed on the line to the date the purchase order has to be released. It treats supplier lead time and safety buffer as two separate legs, because they answer different questions: the lead time is what the supplier needs, and the buffer is what you keep for yourself.
Splitting them is what makes the output usable. The tool reports the latest possible order date — the point at which the material arrives exactly when needed and any slip is a line stoppage — and separately the recommended order date, which is the latest date minus your buffer. Ordering on the latest date is not a plan, it is a bet on a supplier hitting their quoted lead time exactly; ordering on the recommended date is the plan. Seeing both lets you make that trade knowingly rather than discovering the difference when a shipment slips two days.
Both legs count in business days, on the reasoning that suppliers quote lead time in working days and production and goods-in do not run at weekends. Set the work-week and country to match the supplier’s calendar rather than your own where they differ — a European supplier’s August or a Chinese supplier’s New Year will move the date far more than any buffer you set, and neither appears on your domestic holiday calendar. The tool also flags whether the recommended date has already passed, which is the first thing to know when you are triaging a shortage list. What it does not model is order quantity, economic order quantity, minimum order quantities or supplier capacity constraints: it answers when, not how much.
Worked example
A part is needed on the line on Tuesday, 30 June 2026, with a supplier lead time of ten business days and a five-business-day safety buffer. The latest possible order date is Tuesday, 16 June — order then and the material lands exactly on time, with no room for a slip. The recommended order date is Tuesday, 9 June, which absorbs a week of supplier lateness without touching the line. Both are shown, so the decision to spend or keep that buffer is an explicit one.
Frequently asked questions
What is the difference between the latest and recommended dates?
The latest possible order date assumes the supplier hits their quoted lead time precisely, so any delay at all becomes a line stoppage. The recommended date subtracts your safety buffer, giving that many business days of absorbed lateness before the need date is threatened. Use the recommended date for planning and the latest date only to know how much runway remains when something has already gone wrong.
Whose calendar should I use, mine or the supplier’s?
The supplier’s, for the lead-time leg — that is the calendar the material is being made against. This matters most for the shutdowns that do not appear on your own holiday list: a European supplier’s summer close-down, Chinese New Year, or an Australasian Christmas break will each move a delivery date by more than a typical safety buffer covers. Where the two calendars differ materially, run the tool twice and plan to the worse answer.
How big should the safety buffer be?
That is an inventory-policy question this tool does not answer, but the usual basis is the supplier’s observed delivery variability rather than a round number — a supplier whose lead time varies by a day needs a smaller buffer than one that varies by a week, even if both quote ten days. Set it from your own on-time-delivery data where you have it, and revisit it when a supplier’s performance changes.
What this calculator does not do
A limitation you know about costs far less than one you find after the deadline. These are the specific things this tool cannot work out for you.
- Lead time and safety stock are figures you supply. It does not model demand variability, which is what safety stock actually exists to absorb.
- Supplier lead times are counted in working days on your calendar, not the supplier’s. A supplier in another country keeps different holidays.
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