The Canon T90 is equipped with an internal back-up battery (3V lithium button cell). It powers memory and retains stored input data.
The official Canon support time for this camera has long expired and so it is very difficult to find a suitable spare part. The original spare part is attached with solder tabs which third party batteries will lack.
If you're favoured by fortune and find one (e.g. dismount from a broken camera) you can replace your old battery by self-made work, if you're a skilled electronic technician.
Do it yourself and at your own risk!
Remark: The camera body disassembly and reassembly is shown by my former video "Canon T90 Modification - Film Leader Stays Out How-To Tutorial"
