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From: c28f62@TheWorld.com (Mark Kramer)
Newsgroups: comp.os.linux.misc,comp.sys.raspberry-pi
Subject: Re: Keeping 'order' without RTC.
Date: Tue, 12 May 2015 17:29:57 +0000 (UTC)
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In article <mit74b$338$1@dont-email.me>,
William Unruh  <unruh@invalid.ca> wrote:
>No, there is no designation of a leap second on Unix time. Unix time
>will never return the designation above. It simply stops the clock for
>that second making 23:59:59 two seconds long, not one.

The system clock does not stop "for that second". The system clock has no 
concept of leap seconds or when they are going to happen. If you have
no time management other than the system clock, your system clock will be
off by a second after a leap second occurs.

That's where NTP comes in.