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Because it needs to be running at the time that "setclk" is run by crontab (3:01 AM by default). So if you are in the habit of not leaving your machine running at night, the change will not be made.
John Dubois explains: Because the time reported by the CMOS (motherboard) clock is *local time*, and local time includes the effects of DST. It has to, because the BIOS interprets it this way; if the OS tried to leave the CMOS clock set to non-DST time, the BIOS would report the wrong time and admins would "fix" it when they noticed the wrong time being reported.
So, if the system is booted and sees that it's July and 15:10, then it's July and 15:10. If it's booted and sees that the time is November and 15:12, it's November and 15:12. There is no attempt to shift time from what the CMOS clock reports. The CMOS clock is kept in sync with local time by adjusting it back or forward as neccessary when the DST transitions occur, via that /etc/setclk command in crontab. But this obviously only occurs if the system is running at the right time.
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