It doesn't happen every time I change batteries - the main battery - it's happened maybe 3 times since I got the camera a couple weeks ago. The date/time battery is rated for 5 years and I assume it's the original.
Only thing - I figure when a battery dies it dies. But I can set the correct date just fine and it'll stay for...a few days? a week? I don't know it's pretty random.
Is this the sign of a failing battery you think?
(it's the 5D not the MkII)
Thanks you guys!