Hi Kevin ... I found this insight into it ...(via here - https://en.wikipedia.org/wiki/Doomsday_Clock#Basis_for_settings)

The Clock's setting is decided without a specified starting time. The Clock is not set and reset in real time as events occur; rather than respond to each and every crisis as it happens, the Science and Security Board meets twice annually to discuss global events in a deliberative manner. The closest nuclear war threat, the Cuban Missile Crisis in 1962, reached crisis, climax, and resolution before the Clock could be set to reflect that possible doomsday.