I had an issue with
ntp1.dlink.com or
ntp.dlink.com.tw NTP server, that is whenever I clicked on "Reboot the device" button, DIR-615 failed to synchronize time with NTP server after reboot. It was quite irritating because all your schedules would fail without accurate time.
Now I set NTP server as
129.6.15.28 (anyone can use this ntp server worldwide) and it's perfectly working and DIR-615 synchronizing time without any problem. Now I'm happy

.