This maritime ETA calculator estimates a vessel's arrival time accounting for time zone differences between the departure and destination ports. It calculates ETA in UTC, local destination time, and ship's clock time — and provides step-by-step clock adjustment instructions including International Date Line crossing detection.

1. Enter the departure date (DD/MM/YYYY) and time (24-hr).
2. Select the departure port's GMT/UTC offset.
3. Enter the voyage distance and vessel speed.
4. Select the destination port's GMT/UTC offset.
5. Click Calculate ETA.

Departure: Singapore (GMT+8), 08:00, 25 Sep 2026
Distance: 1,200 NM at 14 knots
Destination: Dubai (GMT+4)
Transit time: 85h 43m → ETA: 29 Sep 2026 21:43 GMT+4
Clock adjustment: RETARD by 4 hours (westbound)

The calculator assumes constant speed throughout the voyage. Real passages are affected by currents, weather, traffic, port waiting times and speed reductions. Clock adjustment instructions follow standard maritime practice — retard (set back) when travelling westward, advance (set forward) when travelling eastward. Always use official charts, weather forecasts and GMDSS information for actual navigation.

• Using maximum speed instead of average sea speed — allow for manoeuvring, traffic and weather.
• Selecting the wrong GMT offset for the departure or destination port.
• Forgetting to account for International Date Line crossings on trans-Pacific voyages.
• Ignoring tidal windows for port entry.

What is a typical merchant vessel speed?

Container ships: 18–25 knots. Bulk carriers: 12–15 knots. Tankers: 12–16 knots. For planning purposes, use the vessel's service speed minus 10–15% for weather and operational factors.

What does 'retard' and 'advance' mean for the ship's clock?

Retard means set the clock back (westbound travel). Advance means set the clock forward (eastbound travel). The total adjustment equals the difference in GMT offsets between departure and destination.

What happens when crossing the International Date Line?

Crossing the IDL westbound (e.g. Pacific to Americas) gains a calendar day — the same date is repeated. Crossing eastbound loses a day — one date is skipped entirely. The ship's log and official records must be updated accordingly.

What is Ship's Clock Before Adjustment?

This shows what the crew's clock would read at arrival if they never changed it from the departure time zone — useful for planning when clock changes should occur during the voyage.

Does this account for Daylight Saving Time?

No. The calculator uses fixed GMT offsets. If the departure or destination port observes DST, manually adjust the GMT offset to reflect the correct offset at the time of departure or arrival.