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 12V 500mA from the power supply time is running slowly i e : Normally 1 minute = 60 seconds, but it takes 150 seconds. I checked voltage regulator by measuring its output voltage.It is perfect.What will be the problem ?Please help.Thanks in advance.

Noise affecting the RTC perhaps - the crystal oscillator on an RTC is extremely sensitive
to electrical noise as it runs at ultra low power. Try using an external 5V regulator
and keep the 12V away from the RTC - linear regulators output much less noise on
the output rail.