date is way off when I format a sd card..

For sure! The PRAM battery is what holds the date info and runs the clock when you have the system turned off if you have unplugged it. A common issue is you use an outlet strip with a power switch so the vampire power the iMac uses to draw the power to keep the clock going is gone so the PRAM battery is then used.

And of course! This is an old system so the battery just from age can runout as well.

For now just go to your date and time settings to fix the clock and as long as you just leave the system on in sleep mode the date and time should stay correct.

iMac Intel 20" EMC 2133 und 2210 PRAM Batterie austauschen