On June 16, 2023, between 17:00 - 17:30 US Eastern Daylight Time, the Maryland NIST servers seem to have gone offline. I set chrony to include IPv6 servers in both Fort Collins and Boulder, Colorado. I also set my cron job that checks time to only point at the 4 IPv6 Colorado servers.
When the Maryland servers are working, I expect 90% of polls to show lower than 0.4 ms offset. The offset to Colorado seems to be 1.0 - 1.1. So, I've set my alarm tolerance from 1ms to 1.4ms.