Do arrival and departure days count?

The arithmetic that surprises people is not complicated, it is just not what a calendar suggests. Fly in on Friday evening, fly out on Monday morning, and you have spent three nights in the country — but under an "any part of a day" rule you have used four days. Friday counts because you were there for part of it. Monday counts for the same reason.

Do that once a month and the gap compounds: twelve trips of three nights each is 36 nights, but 48 counted days. Against a 90-day limit that is the difference between comfortable and over.

"Any part of a day" is the overwhelming norm — the great majority of rules in this guide use it, and the exceptions are specific enough to name individually. If the rule you are tracking is not one of these, assume both travel days count.

• The UK counts nights, not days. Under the Statutory Residence Test the question is whether you were in the UK at the end of the day — midnight. Arrive on Friday and leave Monday morning and the UK counts three days, not four, because you were not present at midnight on Monday. There is a deeming rule on top, though: if you were UK resident in any of the previous three tax years, have at least three UK ties, and spend more than 30 days in the UK without being present at midnight, every such day after the first 30 counts as a day spent in the UK. For anyone in that group the three-day answer above becomes four.
• China counts full 24-hour days. A day only counts if you were present for the whole of it, which removes both travel days from the count.
• Cyprus does not count the departure day. The arrival day counts, the departure day does not — so a trip arriving Monday and leaving Friday is four days in Cyprus, not five. A same-day departure and return counts as a day outside Cyprus.
• Virginia excludes transit. Passing through without stopping does not create a day.
• Portugal, Taiwan and Ohio count nights rather than calendar days, each with slightly different wording.
• Absence rules invert the question. UK ILR and US green card rules count whole days of absence, so your travel days usually count as days present, not days away — the opposite of the tax-residency convention, and it catches people who apply one habit to both.
• Canada inverts it again. Permanent residence is a presence requirement rather than an absence cap — 730 days physically in Canada in any rolling five years — with any part of a day counting as a day present.

One trip can be counted two ways at once, and this is the single most common source of a wrong number.

Your Schengen 90/180 allowance is an immigration count: any part of a day inside the area counts, including the day you cross the border. Whether you became tax resident in Spain or Portugal is a separate question, answered by that country's own rule with its own counting method and its own window.

The two do not share a total, do not share a window, and can disagree about the same day. Track the day once and apply both rules to it — do not assume the stricter one covers you.

• HMRC — Residence and FIG Regime Manual: meaning of a day spent in the UK
• HMRC — RFIG20720: the deeming rule
• IRS — Substantial Presence Test
• European Commission — Short-stay visa calculator and the 90/180 rule