Since the offset is inconsistent and the clock issues seem to revolve around the system not being powered on it could be a CMOS battery issue. If it were an NTP (port 321) or timezone issue you would likely see a consistent offset. None of these symptoms are typical of an overheating system.
Nonvolatile BIOS memory refers to a small memory on PC motherboards that is used to store BIOS settings. It is traditionally called CMOS RAM because it uses a volatile, low-power complementary metal-oxide-semiconductor (CMOS) SRAM (such as the Motorola MC146818 or similar) powered by a small "CMOS" battery when system and standby power is off. The typical NVRAM capacity is 256 bytes.
The CMOS RAM and the real-time clock have been integrated as a part of the …