Hi,
... and the service will actually accept incoming connections?
You could use
this UDP port scanner to check if your DIR WAN port possibly accepts unsolicited incoming UDP messages directed to the NTP port 123/udp (when saying "unsolicited" I mean: Of course it will accept replies received as a result to requests it has sent to external NTP servers before in order to synchronize local time) . Before doing so, please read the section titled "How it works" to understand the subtleties of UDP port scans ...
...the vulnerability is in the incoming (server) part of the NTP daemon. The firmware page of my router lists the time service as a server. Do you know if this is accurate...?
NTPD (if at all used by your D-LINK box - this question and if a vulnerabilty exists can only be answered by
D-Link's Security Advisories site - the latest advisory
SAP10048 (published 2014/12/22) applying to DIR-655 Rev. Bx says nothing about time service vulnerabilities) can be both a client and a server (depending on configuration): It acts as a client when asking external NTP servers for time synchronization and it possibly acts as a server for devices inside your LAN if they are configured to use your DIR as a time source (don't know if DIR-655 supports this use case).
As far as I understand the vulnerability (and I may be wrong) it is also effective if NTPD operates as a client only (insofar it may receive "bad" replies from attackers using spoofed IP addresses), but only if it is configured to use cryptographic authentication functions (which it is not by default).
PT