How to use the sea time calculator
- Add each contract with its sign-on and sign-off dates, exactly as they appear in your discharge book (CDC) or sea service testimonials. The vessel name is optional.
- Check the totals. The calculator counts both the sign-on and the sign-off day, merges any overlapping dates so no day is counted twice, and converts the total into months and years.
- Pick a period to see how many of those days fall inside a calendar year or an April–March financial year, which is useful for tax and residency questions.
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How sea time is counted
Most maritime administrations count sea service in days and convert it with a 30-day month. Some count whole calendar months first and pool the leftover days at 30 to a month. Many also limit how much standby, yard or training time can count. The result here shows the 30-day convention alongside calendar averages, so you can match whichever your administration uses.
What the sea time is used for
- Certificate upgrades. Under the STCW Convention, for example, a master's certificate for ships of 3,000 GT or more needs at least 36 months of approved seagoing service as an officer in charge of a navigational watch, or 24 months if at least 12 of them were served as chief mate.
- Revalidation. To revalidate a certificate of competency every five years, STCW asks for at least one year of seagoing service in the preceding five years, or one of the alternative routes such as a test or an approved course.
- Tax and residency. Many countries look at days outside the country in a tax year. India's financial year runs from April to March and the UK's tax year from 6 April, for example. Use the period filter, then check the exact rules with a tax adviser.
Worked example
The three example contracts are counted like this:
| Contract | Sign-on | Sign-off | Days |
|---|---|---|---|
| Bulk carrier | 10 Jan 2025 | 20 Jul 2025 | 192 |
| Container ship | 5 Oct 2025 | 18 Apr 2026 | 196 |
| General cargo | 15 Jun 2026 | 31 Oct 2026 | 139 |
| Total | 527 |
527 days is 1 year, 5 months and 17 days with 30-day months, or about 17.3 calendar months. Of those days, 247 fall in calendar year 2026: 108 on the container ship from 1 January and all 139 on the general cargo ship.
Keep your records straight
- Make sure every sign-on and sign-off is stamped in your discharge book and matches the ship's crew list.
- Ask for a sea service testimonial from the company or master when you sign off.
- Keep copies of contracts. Overlapping or missing dates are the most common reason applications are delayed.
SENA's crewing department mans our managed fleet with qualified, competent seafarers. Studying for your next certificate? The squat and FWA/DWA calculators cover two favourite exam topics.