How does a 30-day lockdown solve anything? Assume we do that and it works perfectly. What stops CV from coming over from other countries on day 31?
It's OK if new
isolateable cases come in starting on Day 31. IOW, the 30 days off gives America a kind of a do-over -- a chance to institute isolation measures and contact tracing that should have been initiated mid-February.
It's not so much a blanket problem of "having cases". It's having cases running wild in the background. A month pretty much burns out those "wild cases". On Day 31, you catch them as they come in as best you can (yes, some will be missed in real time ... but hopefully only a small number of asymptomatics).