Bios time not updated

I have asus m2n68 motherboard and date and time not updated though i replace battery. it show last boot date and time
eg: as i use System at 14/1/2015 18:20 and i power off
and after two days when i power on thae date and time shows 14/1/2015 18:20

I updated Bios and replace cmos battery but still the problem is not solve