This is NOT specific to your computer, but will work with any brand and model. The first is OS independent.
If it don't keep time when OFF, you need to replace the BIOS battery. It's a flat, button style battery that is very easy to replace. Although it's not recomended, that battery can be changed when the computer is powered up. It's the ONE AND ONLY piece inside any computer that can be hot replaced.
When that battery is dead, or you just replaced it, you should get a prompt as you power up to manualy set the correct date and time. Just enter the correct date and time in the fields.
When Windows is up and running, you can click the clock on the task bar. It will bring up a popup from where you can adjust the date and time.
You can also right-click the clock and chose: "Adjust date/time". This will allow to set the date, time and time zone.
If you use Linux, you get a similar functionality in about the same maner.
Setting the date and tiem in the OS will also set it for the BIOS.