I got a 671 from jujudeals.com and the exact same thing happened. When I talked to them, they said to first, go into clock mode. While you're in clock mode, press and hold select (SEL). Still holding select, use the Fast Forward and Reverse buttons to change the time. FF will change the hour, REV will change the minute. Hope that helps, and if not, you can contact the Naxa support people at this page: http://www.naxa.com/newsite/support.html
Answers & Comments
I got a 671 from jujudeals.com and the exact same thing happened. When I talked to them, they said to first, go into clock mode. While you're in clock mode, press and hold select (SEL). Still holding select, use the Fast Forward and Reverse buttons to change the time. FF will change the hour, REV will change the minute. Hope that helps, and if not, you can contact the Naxa support people at this page:
http://www.naxa.com/newsite/support.html
Just a note on the solution by social385, clock mode is just when the power is off. :)