Detention Is Not Demurrage

Container free time, demurrage, billing deadlines, bill of lading dates and EDI timestamps.

Detention Is Not Demurrage

A container is discharged, made available, gated out and returned empty. Which two instants does the detention clock run between?

The naive approach

// The naive approach: one clock, from the first event to the last
const discharged = new Date("2024-06-14T19:00:00Z");
const emptyReturned = new Date("2024-06-27T09:00:00Z");
const days = Math.ceil((emptyReturned.getTime() - discharged.getTime()) / 86400000);
// 13 — six of those days were demurrage at the terminal, not detention on the road

Watch it break

The naive approach silently produces wrong results — there is no GMT equivalent to demonstrate because the failure is not using GMT.

Why

On an import, demurrage runs from discharge until the container leaves the terminal gate, and detention from gate-out until the empty is returned. On an export the same two charges run from the full gate-in to loading, and from the empty pick-up to the full gate-in. Terminal storage runs over the demurrage window but is the terminal's own charge, with its own free time, and many carriers also sell a combined clock that runs both as one period. `demurrageClock` picks the start and end events for the charge and direction you name, in any event order, and returns a half-open interval in your own strings that `chargeableDays` takes directly. A required event that is missing, or present twice, returns `null`, because a container discharged twice is a data error and not a choice.

The gmt approach

import { demurrageClock } from "@northguild/gmt/intermodal";

const events = [
{ type: "discharged", at: "2024-06-14T19:00:00Z" },
{ type: "available", at: "2024-06-15T12:00:00Z" },
{ type: "gatedOut", at: "2024-06-20T14:30:00Z" },
{ type: "emptyReturned", at: "2024-06-27T09:00:00Z" },
];

demurrageClock(events, "detention", { direction: "import" });
// { start: "2024-06-20T14:30:00Z", end: "2024-06-27T09:00:00Z" }

demurrageClock(events, "demurrage", { direction: "import" });
// { start: "2024-06-14T19:00:00Z", end: "2024-06-20T14:30:00Z" }

// A tariff that starts the import clock at the availability date.
demurrageClock(events, "demurrage", { direction: "import", startEvent: "available" });
// { start: "2024-06-15T12:00:00Z", end: "2024-06-20T14:30:00Z" }

Same widget, working

Remove the emptyReturned event above and the detention clock returns null: the container is still out, and there is no end to count to. Ask for "storage" and you get the demurrage window again, because the terminal’s charge runs over the same two events. Ask for "combined" and you get discharge to empty return, the merged clock. Feed the detention result to chargeableDays(start, end, freeDays, terms) with the detention tariff’s own free days and counting terms. The full rules are in Intermodal: Free Time and Demurrage.