A long time ago I bought several Lenovo Thinkpad T420 laptops (used of course). But my wife's laptop gave her the dreaded error (after power loss):
Real Time Clock Error - Check Date and Time settings. Press Esc to continue or F1 to enter Setup.
Finally I got fed up with it, so I swapped out her old CMOS backup battery with a new one. Then I also did the same for my other T420's.
iFixit has a good write up on replacing the keyboard on a Thinkpad T420, which can be followed until "Step 6 - Disconnect the keyboard". At step 6 in the 3rd picture, you can see a yellow, shrink wrapped, circular item. That is the CMOS backup battery. It can be easily be unplugged and taken out.
You can buy a new one on ebay starting from $7.99, but you can also try to replace it yourself with a CR2032 button cell battery. I bought an 8-pack of CR2032 batteries for 0.99 euro and grabbed a sharp/thin knife, electrical tape and pliers. Essentially these were the steps:
- slice open the yellow plastic
- remove the battery from the plastic
- use the pliers to remove the leads from the battery
- reapply the leads to the new battery and keep them in place with eletrical tape
- reapply the yellow plastic case and use electrical tape to close it up
After that the laptop can be assembled again using the iFixit manual.