I use the NIST NTP server located in Marylan as the time source for my Stratum 2 servers. I noticed that the last synchronization with the time-a-g.nist.gov server (129.6.15.28) occurred at approximately 10:00 PM UTC.
It seems that all NIST NTP servers located in Maryland are inaccessible, although the status page says otherwise.
NIST servers are also monitored by the NTP Pool, and the monitoring station began to indicate timeout at 21:54:12 UTC.
No response from any Gaithersburg servers, ipv4 or ipv6.
There have been several fiber cuts in recent years.
NIST monitors these servers, but I sent them a note just in case.
Seems to work now from Finland:
$ ntpdate -qu 129.6.15.28
server 129.6.15.28, stratum 1, offset -0.000084, delay 0.12614
9 Aug 14:51:02 ntpdate[41321]: adjust time server 129.6.15.28 offset -0.000084 sec
The pool’s monitoring server seems to think so as well.
Still down from where I am (Netherlands, AS33915):
ntpdate -qu 129.6.15.28
server 129.6.15.28, stratum 0, offset 0.000000, delay 0.00000
10 Aug 10:23:22 ntpdate[9904]: no server suitable for synchronization found
But from many other locations it does respond. I tested it with the NLNOG-ring.
UPDATE: working here now.
There is a manual mailing list. Send a request to be notified to internet-time-service@nist.gov I see that another outage is scheduled:
“There will be a scheduled maintenance outage for the circuit that supports the NIST time servers in Gaithersburg, Maryland. (IP 129.6.15.x). The outage will start at 0400 UTC on 18 August 2021 and will last for up to 5 hours, ending at 0900 UTC.”
NIST isn’t always informed about upstream activities. That’s one reason to have some geographic diversity in the server’s NTP configuration.
Thank you! I will subscribe to the email list to be notified of future interruptions. I use Maryland servers as an important time source for my Stratum 2 servers due to low latency and offset.
If you want more accuracy, then switch from NTPD to Chrony as Chrony is far better at keeping time then NTPD is.
Also, Chrony can run time very accurate even when being offline as it can use RTC as well.
On my systems I have replaced NTPD with Chrony and it ticks like clockwork 
