Canon A590 IS - Manual (Page 186)

184
If the Date/Time in the Set up menu appears when the camera power
is turned on, the date battery charge is low and the date and time
settings have been lost. Please purchase a lithium coin battery
(CR1220) and replace it as follows.
Replacing the Date Battery
The date battery is preinstalled at the factory and may therefore
expire earlier than its rated lifespan after the camera has been
purchased.
Be particularly careful to keep the date batteries out of the
reach of children. Seek medical assistance immediately if a
child swallows a battery since corrosive battery fluids could
damage the stomach or intestinal wall.
1
Confirm that the camera is turned off.
2
Slide the memory card slot / battery cover lock
and open the cover.
3
Hook your nail onto the date
battery holder and lightly
pull it up.
4
Pull out the battery holder in
the direction indicated by
the arrow.