Remove the battery for a couple days.
The (!) note on p. 30 of the manual as much as says that. Like most computers and increasingly more DSLRs, the date/time back-up battery is a cell permanently embedded on its circuit board that recharges from the main battery. It's only intended to hold settings for a short time between battery changes or short term storage. It won't hold settings for long term storage like the button battery cells
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
04/20/2023: New firmware version 1.4.1 is available for EOS R3
03/30/2023: New firmware updates are available.
2/07/2023: New product announcements!EOS R8 EOS R50RF-S55-210mm F5-7.1 IS STMRF24-50mm F4.5-6.3 IS STMRF15-30mm F4.5-6.3 IS STM