The "live" clock (while the PC is running) is just an interrupt driven software counter.
If the interrupt load is too high, IRQs will be dropped, and the clock will go slow. This is rare though, in fifteen years of PC engineering I've seen it once - and that was on a 386/20 file server dinosaur running 14 workstations off 7 MBytes of RAM and a single SCSI disk on an ISA controller
More likely on a modern system is ignorance against sleep modes. The OS, or BIOS, should reload the software driven time from the battery backed RTC when waking up from a powersave mode in which the CPU has been halted. And even if it does, it might still be that the application that displays the time doesn't notice.
Regards, Peter