Enable NTP settings in DD-WRT on the Setup page like this ( EST as example ):

Time Settings
NTP Client: Enable 
Time Zone: UTC-04:00
Summer Time (DST): last Sun Mar – last Sun Oct
Server IP/Name: ca.pool.ntp.org

End result:

root@DD-WRT-INTERNET-ASUS:/tmp# date
Sun Mar 13 15:38:45 UTC 2016
root@DD-WRT-INTERNET-ASUS:/tmp#

Cheers,
TK