My old Canon T2i has stopped remembering the date and time if the battery is removed.
It Turns out there is a hidden internal backup rechargeable “button” battery, not mentioned in the manual.
Luckily it is accessible, as seen here:
https://www.ifixit.com/Guide/Canon+EOS+Rebel+T2i+Clock+Battery+Replacement/139543
The battery is a Seiko MS614SE, not to be found in Canon’s support pages.
After some effort I found a supplier that ships it in the USA:
https://eu.mouser.com/en/ProductDetail/Seiko-Instruments-Micro-Energy/MS614SE?qs=vrQyzjTsotmt1bmGskqEPQ%3D%3D
I will see if the new button battery fixes the problem. There may be more to it.
There is also a similar thing going on with Fuji cameras.
Starting earlier this year, my X-T3 will reset & lose my settings & the date
If the camera is not turned off before removing the battery.
There is a YouTube about it here:
https://www.youtube.com/watch?v=3vOv-xjUtzw
In the X-T3, I do not see an access to a button battery like there is in the Canon T2i.