The short answer
Yes. Each country sets its own residency test, so you can qualify as a tax resident of two (or more) countries in the same period. When that happens and the two countries have a tax treaty, "tie-breaker" rules usually assign you to just one of them for treaty purposes — but you should confirm with a tax adviser or the relevant tax authorities.
Tax residency is decided separately by each country under its own domestic law, and those tests don't line up. The most common trigger is spending 183 days in a place, but the exact day count, the reference period (calendar year, local tax year, or a rolling window), and how a day is counted all vary — and many countries also look at where your home, family, and economic life are, not just days. So it's entirely possible to meet two countries' definitions at once. Examples from this site: the UK treats 183 days as automatic residence (tax year 6 April–5 April) but can also make you resident on fewer days via its sufficient-ties test; Canada deems you resident for the whole calendar year if you sojourn there 183 days or more, unless a treaty says otherwise; New York can treat you as a statutory resident if you keep a permanent home there and spend more than 183 days in the state; and California applies a rebuttable presumption of residency once you spend more than about nine months there, with no bright-line day rule.
Being resident in two countries does not automatically mean you're taxed twice on the same income. Most countries that tax you have a network of double-tax treaties, and a treaty's "tie-breaker" article (modelled on OECD Article 4) steps through a hierarchy — permanent home, then centre of vital interests, then habitual abode, then nationality, and finally agreement between the two tax authorities — to make you "treaty resident" in just one country. Where no treaty applies, you usually rely on foreign tax credits to avoid being taxed twice. These rules are intricate and fact-specific, so dual residents should get professional advice rather than self-assess.
One thing worth separating: a country's tax residency rules are not the same as its immigration or visa limits. The Schengen "90 days in any 180" rule and the new EU border systems (the Entry/Exit System has been fully operational across the Schengen Area since 10 April 2026, and ETIAS is expected to launch in late 2026, with the exact date still to be confirmed) control how long you may stay as a visitor — they say nothing about whether you've become a tax resident. Conversely, you can become tax resident somewhere without ever triggering an immigration limit. Countly tracks day counts so you can see when you're approaching either kind of threshold, but it can't determine your legal tax residency. Always verify your situation with the relevant tax authorities or a qualified adviser; exceptions and special rules are common.
Stop counting by hand.
Countly tracks your days across borders automatically and privately — and warns you before any limit.