One of the things we take for granted is that clocks show the current time.
Before replacing the battery, set the date and time to the correct values in CMOS setup and save and exit the setup.
At the bottom of the time and date window, select Change date and time settings.
In the Date and Time settings window, click the Internet Time tab and then select Change Settings.
Click the clock once to bring up the detailed time and date display, which shows you a mini calendar and analog clock.
You may also see clocks for additional time zones here if you have previously enabled that feature.