As newport boy said, If the battery is dead, it will boot up up with its factory failsafe settings with a date like Jan-1-2002 or whatever. You can buy replacement batteries at any computer store but they will ask stupid money.

Go down to the local recycling centre and get a few from dead computers or motherboards, they will be happy to let you have them for free or 50p. Easy to extract with a small screwdriver (there is usually a clip at one side - push it back and the battery pops out).

Tip: Don't put the batteries in your pocket, small change shorts them out and sets your trousers on fire ... ;O)

AP