I don't remember seeing that in this thread and I've been reading it the whole way. Maybe I missed it at some point?
Anyway, the official narrative from everywhere is that the first case was on January 19:
On January 19, 2020, a 35-year-old man presented to an urgent care clinic in Snohomish County, Washington, with a 4-day history of cough and subjective fever.
https://www.nejm.org/doi/full/10.1056/NEJMoa2001191
China did not report any cases until December 31.
If two people I personally know (who have nothing to do with one another, and live 400 miles apart) had it in early January, it means that this was far more widespread in the US at the time than just a few isolated cases which hadn't yet been recognized.
I suppose it's possible that both just had a bad flu. However, both reported it to have been the sickest they ever were.
This could actually be a huge game-changer if it turns out that a shitload of us already had it and were asymptomatic (or had symptoms so mild we forgot them), and the people we're seeing sick/dead are just the "unlucky" part of the population who suffered major issues from it.
Of most interest, 46.5% of the passengers on the Diamond Princess who tested positive, most of whom were most likely older, had no symptoms.