Hardware RTC(special chip on your motherboard) is used at kernel startup. Then software based RTC is used(because you can attach software interrupts to it) and

Hardware RTC(special chip on your motherboard) is used at kernel startup. Then software based RTC is used(because you can attach software interrupts to it) and this one drifts if user or system changes main system clock BCLK(sometimes called FSB). It is normal for clock to drift 10s for a period of one week, but not for period of five minutes. Example from article: By underclocking the BLCK of a Haswell system from 130MHz to 122MHz (-6%), Windows 8 loses 18 seconds over a five minute period; and the inverse applies to overclocking, too. This does not apply for Windows 7, which adjusts itself fine.