Currently our ntp-service uses the top-level NTP pool servers [0]:

(define %ntp-servers
  ;; Default set of NTP servers.
  '("0.pool.ntp.org"
    "1.pool.ntp.org"
    "2.pool.ntp.org"))

However, the NTP Pool project requests that operating system vendors
(that's us!) don't use these servers directly [1]:

------
You must get approval from the server operator before you hardcode any
IP addresses or hostnames. This is easy to get if your own organization
runs the NTP servers you are planning to use. In most other cases you
will not get it.

Do not use the standard pool.ntp.org names as a default configuration in
your system. The NTP Pool can offer services for you, but it must be
setup in advance (see below).
------

They recommend we apply for a vendor zone instead [2]. We would not have
to run our own NTP servers.

Does anyone know if GNU or the FSF have a vendor zone we could use?
Otherwise, we should apply for one and adjust our ntp-service.

[0]
https://git.savannah.gnu.org/cgit/guix.git/tree/gnu/services/networking.scm?id=ec5c24f9a8421172a0cf9133d512014b31815a76#n336

[1]
http://www.pool.ntp.org/en/vendors.html#basic-guidelines

[2]
http://www.pool.ntp.org/en/vendors.html#vendor-zone