Why does my ETA keep changing?

You booked a reefer of blueberries against a 12 March arrival. It became the 14th, then the 13th, then the 17th, and nobody told you why. Meanwhile the cold store slot is booked, the customer is expecting the fruit in week 11, and you have to decide today whether to move either.

The frustrating part is that none of those revisions was wrong. An ETA is not a commitment that keeps getting broken — it is a forecast being recalculated, and understanding what it is calculated from tells you which movements matter and which are noise.

What an ETA is actually made of #

The date you were given at booking and the date you get in week three are different kinds of object wearing the same label.

At booking, it is a schedule. The carrier publishes a service rotation — this string of ports, this transit, sailing weekly — and your ETA is arithmetic on that rotation. It assumes every call runs to plan for a voyage that has not started. It is a plan, and it is the number your contract was written against.

Once she sails, it becomes a forecast. Now it is recalculated from where the vessel actually is, how fast she is going, and how long her remaining calls are expected to take. This is where the movement comes from: the inputs change daily.

In the last stretch, it becomes a berth prediction. The question stops being “where is the ship” and becomes “when will the terminal work her”, which depends on the queue ahead of her rather than on anything she does.

Three different calculations, one field on your screen. Most systems do not tell you which one you are looking at, which is why the number seems to wander for no reason.

What actually moves it #

Speed. Fuel is the largest variable cost of a voyage, so lines adjust speed to suit fuel prices, schedule buffers and berth windows. A vessel slowing from 20 knots to 14 is not in trouble; she is being operated. But an ETA built on her average will drift.

Port time. Every call is an estimate of hours alongside. Congestion at any port on the rotation pushes everything downstream, including your discharge — which is why an ETA can move because of something that happened at a port your cargo will never see.

Berth queues. A vessel arriving on time and waiting four days at anchor has arrived on time. The ETA that matters to you did not.

The rotation itself. Calls get added, dropped or reordered. A sailing gets blanked and cargo rolls to the next one.

Transshipment. The big one. After a transfer at a hub, the vessel’s ETA and your container’s ETA are different numbers, and only one is about your fruit. A missed connection moves your arrival by a full service interval — seven days on a weekly service — while the original vessel arrives exactly on schedule, reporting beautifully.

Cut-offs at the other end. Documentation and VGM cut-offs sit days before departure. Missing one does not move the ETA; it moves your cargo to a different sailing, which arrives with a different ETA that looks like a delay in your system.

The data quality problem underneath #

Some ETA movement is real and some is an artefact of how the number reached you.

An ETA can come from the carrier’s own systems, from a tracking provider’s model, or from a field a crew member typed into the vessel’s AIS transponder — and that last one is free text nobody validates, frequently stale, sometimes left over from the previous voyage. An ETA sourced that way can “change” because someone on the bridge updated a field, with nothing having happened at all.

Worse, a provider filling gaps between position fixes will recompute an ETA from an interpolated position — an estimate built on an estimate. Why tracking positions are often wrong walks through where that creeps in and how to spot it.

So the first question about any ETA change is not “what happened” but “what moved” — the vessel, or the number.

Which movements deserve a decision #

Most revisions are noise. A few are signals, and they have shapes worth recognising.

A jump of exactly one service interval — seven days, fourteen days — is almost never a delay. That is a rollover or a missed connection, and your container is on a different vessel. Treat it as a different shipment.

Repeated drift in one direction is the most useful pattern on this page. An ETA that has slipped three times in a week will slip again. That is visible days before the final number is, and it is the point at which re-planning is still cheap.

A sudden improvement deserves as much attention as a delay. An ETA moving earlier shortens your window: free time starts at discharge, so a vessel arriving three days early hands you three fewer days, and nobody sends a correction.

Movement inside a day or two, on a long voyage, is weather and speed. Watching it is a waste of attention.

The habit worth building is to stop reading the ETA as a number and start reading it as a series. One value tells you almost nothing. Its history tells you whether the shipment is behaving.

And when the vessel finally arrives, stop using the ETA entirely: the event that starts your clocks is discharge, not the forecast that predicted it.

What it costs to find out late #

The ETA is a forecast about a perishable asset, so a revision is not just a scheduling nuisance — it moves days out of the part of the journey that sells fruit. Transit time is shelf life covers the arithmetic.

Which makes the timing of the information the whole game. A five-day slip you learn about in week one is a planning problem: the ripening slot moves, the customer is told while there is still goodwill, the fruit with least life left goes to the nearest buyer. The same five days learned at the quay is a disposal problem. That gap — between an ETA that changed and an ETA you noticed changing — is what tracking is actually for.

FAQ #

Is a changing ETA a sign that something is wrong? #

Usually not. An ETA is recalculated continuously from speed, port time and berth availability, so small movements are the system working. What is worth attention is the pattern — repeated slips in one direction, or a jump of exactly one sailing interval.

Why did my ETA jump by a whole week? #

That shape is almost always a rollover or a missed transshipment connection rather than a slow voyage. Your container is waiting for the next service, and on a weekly rotation that is seven days. Check for a load event on the vessel that has already sailed.

Should I plan against the ETA or wait for the arrival? #

Plan provisionally against the ETA and commit against the events. Book the haulier and the cold store slot on the forecast, but confirm them on discharge — because free time starts at discharge, and an early arrival costs you days just as surely as a late one.

Can I trust the ETA shown on a tracking website? #

It depends where it came from, and most sites do not say. A carrier milestone is a record; a provider’s ETA is a model; an AIS-sourced ETA is a free-text field typed on the bridge and often stale. When two sources disagree, the carrier’s is the one to believe.