I don't have exact date timelines, but I know the large company I work for decided that the US was a coronavirus hotspot somewhere around the end of February and the beginning of March and banned business travel to or within the US. It was only in mid-March that they shut down the offices themselves, a couple of days before state or local governments began locking down.

I believe the current estimate is that there was uncontrolled community spread in the US by late February, and closing the borders to international travelers wouldn't have done anything to prevent its spread in the US.

I was at a relatively smallish (maybe a few hundred people) tech event in Phoenix the first week in March. There had been some consideration to canceling but they went ahead. They were cleaning surfaces, doing elbow bumps instead of handshakes but no masks, distancing, etc.

I was supposed to head on to another fairly small event in Tahoe the next week but the Thursday before, they canceled it. Which TBH seemed excessive to me at the time. (Though CA was starting to see an increase in cases.) I got home, did some shopping before everything went crazy but about a week later offices were closing, etc.

The Chinese were low key buying up as much medical supplies in Europe as they could get their hands on and flying them to China. This didn't raise any alarms.

The West displayed an enormous level of incompetence same as the Chinese authorities but you can at least understand why the CCP cadre in Wuhan was so negligent. There is something in the human brain that makes people incapable of recognizing danger until they experience it.

Yes, that's about when it became politically feasible to close the borders in New Zealand, as well.