The normal deadline for filing is before April 30 of the year following the concerning tax year (calendar year) - if you received the invitation to file, called "Aangiftebrief" later in the year, the  deadline will be mentioned in the letter called "Aangiftebrief" -. However, there are a few things that are good to know.

Extension for filing
It is usually possible to apply for extension for filing until the deadline in the first reminder "Herinnering". If you apply for extension as a private person, the Dutch tax authorities usually provide an extension until September 30 of the year following the concerning tax year. Extension for filing for tax consultants is usually an extension until April 30 of the year after the normal filing deadline (so 1 year of extension).

Interest is charged if you file later than April 30
If you did not receive the letter called "Aangiftebrief" and/or if extension of filing was applied for, you can file your Dutch personal income tax return later than the normal deadline. However, in such situation the Dutch tax authorities will charge interest on top of the amount of payable tax, in case they issue the (preliminary) tax assessment later than July 1 of the year following the concerning tax year.

If the tax return results in a tax refund, the Dutch tax authorities will refund interest on top of your tax refund if they fail to issue an assessment within 3 months from filing of your income tax return.