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 a forecast that keeps being recalculated, not a commitment that keeps getting broken. Once you know what it is calculated from, you can tell 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 number with the same label.

At booking, it is a schedule. The carrier publishes a service rotation (this string of ports, this transit time, sailing weekly), and your ETA is calculated from 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 is now, 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. That is normal operation. 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 can arrive on time and then wait four days at anchor. On paper she arrived on time. Your discharge 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 and its tracking looks perfect.

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. That last one is free text that nobody validates. It is frequently stale, and sometimes left over from the previous voyage. An ETA from that source can “change” because someone on the bridge updated a field, when nothing has 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 explains how that happens and how to spot it.

So when an ETA changes, first ask what moved: the vessel, or only the number.

This is also where a predictive ETA is worth having next to the carrier’s. It is computed from observed vessel movement and the berth queue at the destination rather than from the published schedule, so late in a voyage it often knows about congestion before the line’s own date has been touched. Treat it as a second opinion. Compare it with the carrier’s number, and do not use it to replace that number.

Which movements deserve a decision #

Most revisions are noise. A few are signals, and they follow patterns you can learn to recognise.

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. You can see this pattern days before the final date is known, and at that point 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 more than a scheduling nuisance. A delay uses up days of shelf life that you would otherwise sell. Transit time is shelf life works through the numbers.

This is why the moment you find out matters so much. 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. Closing the gap between an ETA that changed and an ETA you noticed changing is what tracking is 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 mean the system is 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, then work through what to do when your container gets rolled.

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

Plan provisionally on the ETA and commit once the events happen. 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, plan on the carrier’s date, since it is the one the line works to. Late in a voyage, a predictive ETA that runs later than the carrier’s is still worth taking seriously, because it often sees port congestion before the line updates its own date.