NTP Client (Network Time Protocol) is the software client that allows your router to synchronize with an available time server. Your router's current time is listed in the upper right hand corner of the Web GUI when you log in to your router. Certain features such as "Access Restrictions" and the "Traffic by Month" graph will not function if the router hasn't synchronized with a time server. And if it has synchronized but the time has not been adjusted for your time zone, you may see unexpected behavior. You cannot set your time manually, and must do it via NTP. Don't be fooled by the time zone setting located at "Setup" > "Basic Setup" > "Time Settings". This setting alone is only to adjust the synchronized time from the server to match your local time.

As Brainslayer states "If NTP is disabled the time starts at zero which is 1 January 1970".

As mentioned, you cannot just manually set the time on your router and then forget about it. You can, however, set it to synchronize with a public time server using the NTP client and then forget about it. To configure the NTP client, log into the router and go to "Setup" > "Basic Setup" > "Time Settings". Now:

1) Select "Enable" next to NTP Client
2) Choose the appropriate Time Zone for your area
3) Set Summer Time (DST) as appropriate (See Time Zone settings below)
4) Enter the domain name or IP address of the time server you have selected (See Time Servers below)

Don't forget to Save and Apply settings.

How often does NTP synchronize? According to redhawk0, it is "checked at boot up...then after 120 seconds for a second time....that's it."
http://www.dd-wrt.com/phpBB2/viewtopic.php?p=419797#419797

Have the clocks just changed in your area but you find that DD-WRT is still an hour behind/ahead? Open a Telnet/SSH command prompt and run these three commands:

ntpclient pool.ntp.org
stopservice process_monitor
startservice process_monitor

Courtesy: http://www.dd-wrt.com/phpBB2/viewtopic.php?p=271968#271968