All the days of a stay in the Netherlands count for the purposes of applying the 183-day rule
For the purposes of allocating the right to tax salary from employment, a 183-day rule has been included in tax treaties. If a resident of another State is employed by a foreign employer to work in the Netherlands and there is no economic employer or permanent establishment of the foreign employer in the Netherlands, then the Netherlands only has the power to tax if the employee spends at least 183 days in the Netherlands during a 12-month period (or calendar year, depending on the text of the treaty).