Some additional info:
if I manually run OpenNTPD's ntpd, here's the response:
$this->bbcode_second_pass_code('', '
[root@alarm ~]# ntpd -s -d
ntp engine ready
no reply received in time, skipping initial time setting')
I tried this a few times, each time changing the server in /etc/ntpd.conf. Still nothing.
Following the advice at the top of this page
https://wiki.archlinux.org/index.php/OpenNTPD in the red section, I removed OpenNTPD and installed NTPd (Arch package name is ntp). Using the defaults, I started ntpd, and the time synced without issues.