system date & time

I have a HP pavilion dv7 laptop, i have a problem with system date and time.I have tried for so many times to change the date to current date&time but after working with the laptop for sometime the date&time changes to 1:00 and 1/1/2008. I took some advise from a collegue to change the CMOS battery,i did it ,but it does not work.Can you please advise on solving the problem??