How do you calculate sea time from sign-on and sign-off dates?

Formula, assumptions, and limits

Two discharge book entries, 15 January to 20 June 2024 and 20 June to 10 October 2024, come to 270 days on articles once 20 June is counted once: 8 months 26 days in calendar months, or 9 months at 30 days a month. Both figures are correct; which one an application needs depends on the administration that issues the certificate.

Formula

• $S_i$, $E_i$: sign-on and sign-off date of entry $i$, with the entries sorted by sign-on date.
• $D_i$: days of entry $i$ as written, both ends included.
• $C_{i-1}$: the latest sign-off date among the entries before $i$.
• $S'_i$: the first day entry $i$ counts, the day after $C_{i-1}$ where the entries overlap, so no day is counted twice.
• $D$: days on articles, each calendar day once.
• $k_i$: whole calendar months in the counted part of entry $i$, a calendar month running from a date to the day before the same date in the next month.
• $o_i$: odd days left in entry $i$ after its whole months, and $O$ the sum of $o_i$ over all entries.
• $M_{\text{cal}}$, $d_{\text{cal}}$: months and days by the calendar.
• $M_{30}$, $d_{30}$: months and days at 30 days to the month.

Derivation

STCW Regulation I/1, as amended at Manila in 2010, defines a month as “a calendar month or 30 days made up of periods of less than one month”. The calendar method follows that sentence literally: whole months are taken as calendar months, and only the remainders, the periods of less than one month, are counted in days and turned into months at 30. The UK Maritime and Coastguard Agency (MCA) gives the working example in form MSF 4274, 3 January to 5 March as 2 months and 3 days, and adds that odd days “should be added together and reckoned at 30 days to the month”. India’s 1989 engineer examination rules, rule 19, set out the same sequence: complete calendar months first, then the odd days, and the day the articles commence and the day they terminate “shall both be included”.

The 30-day method drops the calendar. The USCG writes it into 46 CFR 10.107 as “Month means 30 days” and “Year means 360 days”, and AMSA publishes the same division with the example 27 + 30 = 57 days, 1 month and 27 days.

Assumptions

• Every day from sign-on to sign-off is counted; the calculator has no way to mark leave taken inside an entry.
• A day on two entries, usually a sign-off from one ship and a sign-on to the next on the same date, is served once and counted once.
• Days between entries are ashore and not counted.
• The calendar method is applied to each entry and the odd days pooled across all entries, the way MSF 4274 asks for a length per voyage.
• Dates are calendar dates. A USCG or AMSA day is 8 hours of work, so an hour-based count needs the hours, not the dates.

Worked example

1. Entry 1, 15 January to 20 June 2024: 17 + 29 + 31 + 30 + 31 + 20 = 158 days, February 2024 having 29. By the calendar, 15 January to 14 June is 5 months; 15 to 20 June are 6 odd days.
2. Entry 2, 20 June to 10 October 2024: 113 days as written. 20 June is already on entry 1, so the entry counts from 21 June: 112 days. 21 June to 20 September is 3 months; 21 September to 10 October are 20 odd days.
3. Days on articles: 158 + 112 = 270.
4. Calendar: 5 + 3 = 8 months, and 6 + 20 = 26 odd days, under 30, so 8 months 26 days.
5. At 30 days: 270 ÷ 30 = 9 months exactly.

Measured against STCW Regulation II/1, 12 months for officer of the watch, the calendar total is 3 months 4 days short and the 30-day total 3 months short.

Edge cases and limits

A calendar month is 28 to 31 days long, so the two methods drift apart as the service grows: 1 February to 31 July 2025 is 6 months by the calendar and 181 days, 6 months 1 day, at 30. February alone shows it best: 1 to 28 February 2023 is a whole calendar month of 28 days, and 0 months 28 days at 30. A sign-on on the 29th, 30th or 31st of a month followed by a shorter month has no “same date next month”; this page takes that month’s last day as the date, as Excel’s EDATE function does, so 31 January to 28 February 2024 is one month. In 2023, with February ending on the 28th, the month from 31 January ends on 27 February, and 31 January to 28 February 2023 is one month and one day. No administration’s text settles that case.

Regulatory basis

STCW Regulation I/1 sets the month; the sea service minimums are in Regulations II/1 to II/5 and III/1 to III/6, in the text of the 2010 Manila amendments, which Directive (EU) 2022/993 reproduces for the EU. The counting rules are each administration’s: MSN 1856 (M+F) Amendment 1, section 9.2, and MSN 1857 (M+F) Amendment 1, section 4.2, for the UK; India’s MS (STCW for Seafarers) Rules 2014, rule 4(31), for the month; 46 CFR 10.107 and 10.232 for the US. The IMO’s review of the STCW Convention and Code, under way at the Sub-Committee on Human Element, Training and Watchkeeping, gives 2029 to 2030 as its target.

Common errors

• Counting sign-off minus sign-on. The default entries give 157 + 112 = 269 days that way, one short: each entry loses its sign-on day, and only 15 January was not already counted on the other entry.
• Adding two entries that share a day. The default entries add to 271 as written; 20 June is one day on board, not two.
• Dividing by 30 for an MCA application. 270 days is 9 months at 30 but 8 months 26 days by the calendar, and MSF 4274 asks for calendar months and days.
• Counting standby or yard time in full. MSN 1856 Amendment 1, section 9.3, caps it at 2 months toward a deck certificate needing 12 months or more.
• Reading the total as service in the right capacity. Chief mate under STCW Regulation II/2 needs 12 months as officer of the watch, not 12 months on board in any rank.

How each administration counts a month

The two methods part company on short months and long runs. 1 to 28 February 2023 is one calendar month on 28 days, while 1 to 30 March is 30 odd days by the calendar, pooled into one month. Twelve unbroken calendar months are 365 or 366 days, 5 or 6 more than the USCG’s 360-day year.

Where India’s 2014 Rules ask more than STCW

India’s Merchant Shipping (STCW for Seafarers) Rules 2014, G.S.R. 546(E) of 30 July 2014, which the Directorate General of Shipping (DG Shipping) applies to every Indian certificate of competency, raise the convention minimum for five certificates:

• Chief Mate (foreign-going), rule 21: 18 months on ships of 500 GT or more, 6 of them as officer of the watch on 3,000 GT or more, against 12 months in STCW Regulation II/2.
• Master (foreign-going), rule 22: 36 months, reduced to 30 with 12 months as chief mate, or pro rata, where STCW Regulation II/2 reduces it to 24.
• Rating forming part of a navigational watch, rule 33: 15 months, or 6 with pre-sea training, against 6 or 2 in Regulation II/4.
• Rating forming part of an engine-room watch, rule 46: 15 months, or 6 with pre-sea or special training, against Regulation III/4’s 6 or 2.
• Chief engineer, rule 37: 36 months, reduced to 30 with 12 months as second engineer, or pro rata, against 24 in Regulation III/2.

The draft MS (STCW for Seafarers) Rules 2026, published by the Ministry of Ports, Shipping and Waterways under the Merchant Shipping Act, 2025, keep the 18 months for chief mate and the 15 for a navigational watch rating, and add “‘Year’ means a calendar year”. The draft carries no G.S.R. number and is not in force.

What counts as sea service

The total is only as good as the days fed into it. The MCA counts service on merchant ships of at least 24 m load line length or 80 GT proceeding to sea, and wants 6 months of it within the 5 years before the application reaches it (MSN 1856 Amendment 1, sections 9.2 and 9.3). For engineers, 75% of the required service must be on a ship under way on sea passage (MSN 1857 Amendment 1, section 4.3), and on ships that use the main engine only now and then, each day under way counts 1.5 times, never more than the time under the crew agreement (section 5.1).

The USCG credits Great Lakes service day for day toward an oceans or near-coastal endorsement, other inland service day for day up to half the requirement, and, on vessels that do not get under way, deck service at three days for one up to 6 months (46 CFR 10.232(b) and (e)).

Where to find each input

The sign-on and sign-off dates are in the continuous discharge certificate (CDC) or seaman’s book, stamped at each engagement and discharge, and in the crew agreement or articles of agreement. Use those dates, not the dates of joining travel or of arriving home: the MCA reckons from engagement to discharge. Where a company testimonial gives the service, its dates should match the discharge book; MSF 4274 asks for the length of each voyage in calendar months and days, which is the per-entry figure in the working under the calculator.