DPNs move fast. The 21-day clock starts the moment the notice is issued....

https://dlf-ne.org/how-do-i-spot-a-lockdown-dpn-before-the-client-wastes-money-on-the-wrong-step/

DPNs move fast. The 21-day clock starts the moment the notice is issued. Distinguishing between a lockdown and non-lockdown notice is crucial for managing your client's personal exposure