What is the deadline for filing the application for the 30% ruling?
The application must be filed within 4 months after the start of the employment as incoming employee in the Netherlands.
If the application is filed later, than the term of the ruling is reduced with the period between the start of the employment and the first day of the month following the month in which the application is filed.
If you are already working in the Netherlands, have the 30% ruling and decide to change employer then you have 3 months time to find a new employer (arrange a contract) and once a start date is confirmed with your new employer, you have 4 months to file the application.
So a late filing will not have the consequence that you are no longer eligible for the 30% ruling but it will reduce the period during which you can use the 30% ruling.