Sometimes it can take some time to set NTP correctly, and sometimes it needs to be within a certain range before it will actually work (also a reboot normally will force a update/query).
I've found that the dlink NTP service a bit buggy, especially on the dir-685. However, the 4500 seems mostly stable. I my personal favorite for NTP services is pool.ntp.org. I use this both professionally and personally as it has a "pool" of ntp servers so if one goes down or are not reachable, the others will respond. Generally there are 3 servers listed.