The clock on my IMac is not keeping the right time.

Are you connected to the Internet? I ask because OS X resets it's internal clock regularly by contacting a time server. Your clock should always be accurate. There is no battery you can change. Open System Preferences -> Date & Time -> select Date & Time tab and make sure the "Set date and time automatically" box is checked. Also make sure the time server is set to the correct one for your location (America, Asia, or Europe).

p.s. I just noticed your signature which says you are using an eMac. If that is still correct then take a look at this document...

http://support.apple.com/kb/HT3005?viewlocale=en_US&locale=en_US

You can indeed change the battery. It might be worth a try,

The eMac was discontinued in 2006 so it's not supported anymore.

Can you find in the console following strings?

Pacemaker

resetting clock

???