Why the qualifying period matters
To claim SED, you need to build an eligible period of at least 365 days. It sounds simple, but it isn't just “365 days at sea.” It's a continuous period that is made up mainly of days spent outside the UK, and it can be stitched together from several trips — as long as the time you spend back in the UK stays within strict limits.
What counts as a day outside the UK
The test is straightforward once you know it: you are treated as outside the UK on any day where you are outside the UK at midnight at the end of that day. So the day you fly home counts as a UK day if you're home by midnight; the day you fly out counts as an overseas day if you're abroad at midnight. Getting this right for every single crossing is what makes or breaks a claim.
Days spent in the Channel Islands, the Isle of Man and the Republic of Ireland generally count as outside the UK — but the onus is on you to show you were genuinely there. A brief step across a border just before midnight won't wash with HMRC.
The half-day rule
This is the part that trips people up. An eligible period can include time back in the UK — you're allowed a normal life ashore — but two limits apply across the whole period:
- The half rule — No more than half the days in your eligible period can be days spent in the UK. Across the whole period, your overseas days must outnumber your UK days.
- The 183-day rule — You cannot spend more than 183 consecutive days in the UK at any point. Do so, and the period is broken — everything before that break no longer counts, and you have to start building a new period from scratch.
Building the period from several trips
Most seafarers don't spend 365 unbroken days away — they work rotations, come home between contracts, and take leave. That's fine. The rules let you combine a run of trips into one long eligible period, provided the days ashore between them stay within the half rule and you never trigger the 183-day break.
Think of it as a running balance: every overseas day builds the period up, every UK day is allowed as long as the overseas days stay in front.
A simple worked example
You're away for 120 days, home for 40, away for 130, home for 30, then away again. The trips link into one eligible period because your UK days never outnumber your overseas days and you never spend 183 days straight in the UK. Once the whole linked period reaches 365 days, you have a valid qualifying period.
What breaks a qualifying period
- Spending more than 183 days in a row in the UK.
- Letting your UK days rise to more than half of the period.
- A gap where you can't evidence being outside the UK when you say you were.
If a period breaks, it isn't the end of the world — you simply start a new one. But any earnings that fall outside a valid 365-day period won't get the deduction, so protecting an unbroken period is worth real money.
The takeaway
The qualifying period rewards two things above all: spending genuinely more time outside the UK than in it, and keeping accurate dates. If you track every departure and return as you go, building and proving your eligible period becomes simple. Leave it until the tax return is due, and it becomes a guessing game — one HMRC may not accept.