i've got two olympus cameras, not dslr but c7xx, and without fail, the date and time setting will reset itself when I swap batteries, or even when I just open the battery cover and close it again. I've gone through the e-500 manual and it seems that the dslr does not have a dedicated battery for the time and date.
I've missed out on some wedding shots when I had to change the battery, and set the date and time on my c740, so olympus's lack of dedicated date and time battey has put me off for good, or until they decide to put in the extra battery.
However, i must say that their cameras take exceptionally beautiful photos, sharp at the edges and very vibrant colours.
OLYMPUS: If you're reading this, don't be stingy and add that extra battery, it's only the size of a quarter!!!!