This isn’t a change in the VAT return deadlines themselves. The standard Dutch VAT filing deadlines still apply (typically the last day of the month following the reporting period). What’s new is the enforcement of a specific start date for submission: the 24th day of the month.
For example, if you’re filing for the Q2 period (April to June), the earliest you can submit your VAT return is 24 June. Submitting on 23 June or earlier will trigger an automatic rejection.