Anyone crossing the Dutch border mid-year, in either direction, lands in a different return from everyone else. The M-form covers the resident and non-resident stretches of the same calendar year, carries its own deadline, and has a reputation for difficulty that no longer entirely matches how it is filed today.
The M-form (M-biljet) is the special tax return for the calendar year you move into or out of the Netherlands — "M" for migratie. If you moved partway through a year, you can't use the normal return; the Belastingdienst (tax office) sends, or expects, an M-form covering both the resident and non-resident stretches of that year, and yes, you generally have to file it if invited. The deadline sits later than the standard 1 May cutoff — usually 1 July of the following year, with extensions available on request. It used to be a paper-only headache, but it can now be filed online through Mijn Belastingdienst, the tax office's own portal, which has cut a lot of the friction. Refunds are common: full-year tax credits and thresholds get applied against only part of a year's income, so the math often tips in your favor. If your situation involves foreign income, a partial-year 30% ruling, or property abroad, it's the one return worth having a tax adviser at least glance over before you submit.