BIOS time and date reset after boot OS.

I have notebook HP Pavilion dv6 2020ev Corei7, and I´m not able to set up correct time and date. When I set date in BIOS and start Windows 7 there is time difference (few minutes sometimes, few hours etc. but every time different). When I turn off nb (also remove battery) time in BIOS is still OK. But when I start Windows 7 there is difference. BIOS time is ok till I change time or date in Windows 7. And this is big problem, when I update time or date in Windows 7, then after new boot BIOS time and date is reset to 00:00 01/01/2009. I´ve tried everything, update BIOS, fresh installation of Windows 7, change time internet sync settings, change the Hard, RAM,... after a lot of research, i decided to change the bios chip next to the "Control Keyboard chip" on motherboard ( i download last Bios version from HP, and program it), problem not solved.
after that i changed the other BIOS chip. problem not solved again.
finally i change the "Control Keyboard chip" (ENEKB3926QFD2) and unfortunately problem still alive.

note: Sometimes, when I work with my notebook date and time changes it go future or past.
i changed the battery on mother board, No effect.

sorry for my english.

please HeLp Me.

thank u andrew, as is saied, i changed the cmos battery on mother board first but Not effect.

again thank u MilesAhead, i changed the I/O chip but not effect.

ok, i will write on security section.
thanks all.

The RTC or Real Time Clock chip on the motherboard is defective. Good info at Wiki on this. This is a small chip normally close to the battery that maintains the system time.