Let me preface this by saying that I am not a canon camera expert or an electronics expert.... I have a Rebel T6 that would forget the time and date every time I'd change the battery. I searched forum after forum and couldn't find a solution apart from sending it in for repair. After looking at the price for repair, I decided it wasn't worth it for an older camera when I could get a newer (used) camera for about the same cost. Still, I couldn't accept that a home remedy for my t6 didn't exist. I remembered something I had read on a forum that it is a capacitor that powers the date and time memory. As such, it is soldered directly onto the board, hence the need to send it in. It occured to me that maybe the capacitor was drained and wasn't charging up. So I turned off the auto shut-down feature and let the camera sit until the main battery drained. I did this twice, and this morning when I changed the battery again...i didn't need to reset the date and time! So far it seems to be ok. I'm hoping that this doesn't turn out to be a temporary band aid fix.
Sounds like good thinking, did you leave the camera in a on state your saying for like a week? Sounds like it did the trick as long as you leave it on for part of a day here or there i imagine it should be fine. Think of it kinda like a car battery where you need to drive actually to keep it topped up, a 5 block drive to the store is not really enough for the alternator to top off battery is loosing a bit of charge slowly just sitting. I did not realize the rebels went this route, even my 20d has a button battery in it.
I actually only put this to the test last night. Since I posted this morning I repeated this process 2 more times- each time with a fully charged battery. Each time I've swapped the battery, the camera booted up without the need to reset the clock. It's lookin' good! I'm hoping that this is also somehow the cure to another problem my camera has which is that a fully charged battery will completely discharge overnight even if the camera not in use. Fingers crossed.
