If you replaced the cmos battery on the motherboard.
And the correct time and date ect are reset when you turn the system off at the power switch.
It will mean that you have the jumper reset cap that bridges two pins on the motherboard set to flush and set the bios back to factory default board settings each time power is lost to the system.
You will have on your motherboard a set of three pins in a line, where the plastic jumper will bridge two pins at any one time Winkebab.
If it is moved over so it bridges the pin showing to the left or right of the three pins.
Two bridged are for normal operation of the bios, where the other two pins if the jumper is placed as said leave the bios in a constant factory default resetting mode of the bios.
So find the jumper in question on the motherboard with the three sets of pins in a line.
And move it over to the left or right by one pin that is currently shown.
If you look carefully on the motherboard it will say cmos clr near the jumper.
Click on the link bellow to see an example of a bios reset jumper and bridge as you should see on your own motherboard.
https://www.google.co.uk/search?q=cmos+bios+reset+jumper&espv=2&biw=2048&bih=1062&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiCtcfVpL3QAhUhM8AKHTxeB5IQ_AUIBygC#imgrc=IFdD0VqMa5dTnM%3A
The jumper is set to normal operation, in the picture.
Where as moving it over to the pin showing would reset the motherboard back to factory defaults.
Resetting the time and date.
The bridge needs to be set back to the left side to ensure the board keeps the correct time and day there after.