Power supply problem

Hi everyone ,
I made a digital clock using Arduino duemilanove w/atmega328 , IN1307 (instead of DS1307 rtc) & 4 CC 7-segment display.The clock was perfect when i powered from USB.But when i supplied a 12V 500mA power time is running slowly.A minute takes more than 150seconds. I checked voltage regulator by measuring its output voltage.It is perfect.What will be the problem ?Please help.Thanks in advance.

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You have not provided enough information to say what is wrong. We need to know more detailes, like what arduino what power supply what time indication.