The key factor in residency is the physical presence for more than 183 days in a fiscal year.
For this reason countries around the world use the 183 days to broadly determine whether to tax someone as a resident.
What is considered as a day of physical presence varies widely under the different domestic laws of countries and a part of the day, a presence at midnight, or a stay of just one minute can count as a full day.
The 183-days test is enforceable in countries that exercise tight control over their borders and taxpayers.
In some countries, the proof is on the burden of tax authorities while in others this burden is on the shoulder of taxpayer.
