The 21-day DPN clock starts on the issue date, not when it hits your desk....

https://www.easybookmarkings.win/when-an-ato-director-penalty-notice-hits-the-21-day-clock-starts-knowing-how

The 21-day DPN clock starts on the issue date, not when it hits your desk. Distinguishing between lockdown and non-lockdown notices is essential for effective client protection. This guide covers the basics, so you can triage fast and keep options open.