Boots up with wrong date - Dell Dimension 3100 PC Desktop

You need to have the battery in order for system to boot, however, you can reset it.

http://www.howtogeek.com/131623/how-to-clear-your-computers-cmos-to-reset-bios-settings/
How to Clear Your Computers CMOS to Reset BIOS Settings

probably set to USB or external as first boot device change in Bios

You can answer Ihm that it is not to trust throug set the correct Date and Time on the Bios!

That means that something is wrong with the hardware or the CMOS battery is dead. Look at the date and time, if it is wrong then you need to replace the battery. If the date and time and are ok, then start checking the bios to see if it lists the hard drives and the RAM. If it does not list them or lists them incorrectly, change them.

If you fixed the Date and Time in the BIOS and when you boot up the laptop next time the Date and Time is wrong, then most like the BIOS battery is flat and needs replacing.
After the BIOS battery has been replaced go to the Bios and set the Date and Time.

If you fixed the Date and Time in the BIOS and when you boot up the laptop next time the Date and Time is wrong, then most like the BIOS battery is flat and needs replacing.
After the BIOS battery has been replaced go to the Bios and set the Date and Time.

When you boot up and the Date & Time is wrong then you correct the Date & Time, then when you shut down and boot up again the Date and Time has changed. This is an indication that the BIOS battery is flat and you need to replace the BIOS battery. Install a new BIOS battery, go into the BIOS setting and set the Date & Time etc. and the new battery will keep your settings stored in the BIOS.

the CMOS battery of your computer is depleted.. Try to change it to save your BIOS setup including your time and date setting.

If this error message comes up when you boot up the computer, it means the BIOS requires configuration, usually the Time and Date is wrong.
If this occurs every time you boot up your computer and have changed the Date and Time then this could be caused by a flat BIOS battery.
If so, replace the BIOS battery and then configure the Date and Time and you won't get this error again.

press F2 and enter BIOS menu - find Exit Menu and select
'Reset to defaults' -

Check the time and date setting in BIOS menu - correct it if wrong.

then Exit and Save.

If time/date was wrong - you may need to replace the internal button cell battery on the mainboard.