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You forget to take into account that almost all systems nowadays are set to synchronize their time to a time server (if they have any internet connection). This means that the RTC is periodically adjusted to whatever the configured timeserver reports (and smarter algorithms will attempt to do this adjustment incrementally rather than in one big step). The timer tick however is simply an internal interrupt timer derived from the crystal oscillator that drives the CPU. This oscillator is not very accurate as it really doesn't matter that much if your CPU operates at 2.210000000000 GHz or 2.21100000000000 GHz. The timer tick is used to drive the RTC in between time server synchronizations but not more.