When a kernel commit starts with "In A.D. 1582 Pope Gregory XIII found that ..." you know you're in for a ride:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=f076ef44a44d02ed91543f820c14c2c7dff53716
tl;dr: Rockchip decided November should have 31 days...
rtc: rk808: Compensate for Rockchip calendar deviation on November 31st - kernel/git/torvalds/linux.git - Linux kernel source tree

@kees wait, a HW manufacturer decided they'd just redefine the calendar? WTF?
@fuzzychef @kees I mean, I'd rather suspect this is a bug that made it into hardware unnoticed rather than an intentional choice, but yes