Canon EOS 40D - Manual (Page 171)

171
The date/time (back-up) battery maintains the camera’s date and time.
Its service life is about 5 years. If the date/time is reset when the battery
is replaced, replace the back-up battery with a new CR2016 lithium
battery as described below.
The date/time setting will also be reset, so be sure to set the
correct date/time.
1
Set the power switch to <2>.
2
Remove the battery.
3
Take off the battery holder.
4
Replace the battery.
Make sure the battery is in the proper
+ - orientation.
5
Insert the battery holder.
Then load the battery and close the
cover.
Replacing the Date/Time Battery
(+) (–)
For the date/time battery, be sure to use a CR2016 lithium battery.