Large number of port 37 (RFC868 time protocol) requests from fixed.kpn.net IPs?

I have a VPS in the Netherlands and I joined it to the pool. After that, I was looking at my firewall log and noticed I was getting several requests per second from various IPs, addressed to UDP port 37, associated with the ancient RFC868 time protocol. All IPs reverse-resolved to “fixed.kpn.net” which seems to be some kind of Netherlands ISP: https://www.overons.kpn/en/kpn-in-the-netherlands/our-network/fixed-network

All of the IPs sending traffic to port 37 are IPV4, although I’m in the pool as both IPV4 and IPV6.

I was feeling generous so I installed xinetd and activated a server for this protocol. (The protocol supports both TCP and UDP but I enabled it for UDP only as that was all I was seeing requests for.)

I allowed the traffic through the firewall & verified that I’m responding to the requests now. I thought if they got a response they might shut up for a while, but it seems like each IP continues to send a new request every few seconds.

Has anyone seen anything like this before or have any insights / ideas about exactly what is happening here?

tcpdump log showing some of these requests and responses:

08:28:06.709097 IP 86-95-86-101.fixed.kpn.net.10740 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:06.709365 IP ipv4.MYDOMAIN.time > 86-95-86-101.fixed.kpn.net.10740: UDP, length 4
08:28:07.047885 IP 86-90-14-93.fixed.kpn.net.65518 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:07.048146 IP ipv4.MYDOMAIN.time > 86-90-14-93.fixed.kpn.net.65518: UDP, length 4
08:28:07.154944 IP 86-86-199-112.fixed.kpn.net.56526 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:07.155230 IP ipv4.MYDOMAIN.time > 86-86-199-112.fixed.kpn.net.56526: UDP, length 4
08:28:07.179140 IP 145-53-219-150.fixed.kpn.net.40518 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:07.179440 IP ipv4.MYDOMAIN.time > 145-53-219-150.fixed.kpn.net.40518: UDP, length 4
08:28:08.013170 IP 86-85-67-163.fixed.kpn.net.35891 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:08.013484 IP ipv4.MYDOMAIN.time > 86-85-67-163.fixed.kpn.net.35891: UDP, length 4
08:28:09.304604 IP 77-168-76-16.fixed.kpn.net.64295 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:09.304838 IP ipv4.MYDOMAIN.time > 77-168-76-16.fixed.kpn.net.64295: UDP, length 4
08:28:09.437496 IP 86-85-67-163.fixed.kpn.net.64939 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:09.437735 IP ipv4.MYDOMAIN.time > 86-85-67-163.fixed.kpn.net.64939: UDP, length 4
08:28:10.909157 IP 86-95-86-101.fixed.kpn.net.10740 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:10.909429 IP ipv4.MYDOMAIN.time > 86-95-86-101.fixed.kpn.net.10740: UDP, length 4
08:28:11.247969 IP 86-90-14-93.fixed.kpn.net.65518 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:11.248203 IP ipv4.MYDOMAIN.time > 86-90-14-93.fixed.kpn.net.65518: UDP, length 4
08:28:11.355008 IP 86-86-199-112.fixed.kpn.net.56526 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:11.355288 IP ipv4.MYDOMAIN.time > 86-86-199-112.fixed.kpn.net.56526: UDP, length 4
08:28:11.379163 IP 145-53-219-150.fixed.kpn.net.40518 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:11.379452 IP ipv4.MYDOMAIN.time > 145-53-219-150.fixed.kpn.net.40518: UDP, length 4
08:28:12.213238 IP 86-85-67-163.fixed.kpn.net.35891 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:12.213489 IP ipv4.MYDOMAIN.time > 86-85-67-163.fixed.kpn.net.35891: UDP, length 4
08:28:13.504423 IP 77-168-76-16.fixed.kpn.net.64295 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:13.504679 IP ipv4.MYDOMAIN.time > 77-168-76-16.fixed.kpn.net.64295: UDP, length 4
08:28:13.637538 IP 86-85-67-163.fixed.kpn.net.64939 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:13.637780 IP ipv4.MYDOMAIN.time > 86-85-67-163.fixed.kpn.net.64939: UDP, length 4
08:28:15.109358 IP 86-95-86-101.fixed.kpn.net.10740 > ipv4.MYDOMAIN.time: UDP, length 0
08:28:15.109674 IP ipv4.MYDOMAIN.time > 86-95-86-101.fixed.kpn.net.10740: UDP, length 4