Mass media article on leap seconds
bww at acm.org
Tue Mar 2 17:08:40 UTC 2004
> Your correspondent's two cents: in setting up the time handling in UNIX, T&R
> got it exactly right with respect to springing forward and falling back when
> DST goes in to and out of effect--keep the computer counting monotonically
> and leave it to the software to translate the monotonic count into a
> representation of local time. What's right at the level of an hour is also
> right at the level of a second--keep the computer counting at one count per
> second, and leave it to software to figure out what should be displayed when
> the user asks what time it is.
> I don't agree. There is the important difference that UTC is today very
> widely disseminated, whereas TAI is a curiosity only known to time geeks
> like us. Keeping a computer synched to something like TAI would only be
> practical in the real world if a leap-free timescale (e.g., the existing
> TAI or GPS time) were widely enough available, along with a regularly
> updated UTC-TAI offset table. Current time distribution services,
> however, provide only UTC in easily accessible form, therefore running
> machines in TAI would likely cause them to get the leap second offsets
> wrong due to out-of-date leap-second tables rather quickly. Their
> timestamps would quickly get an integer number of seconds wrong relative
> to the timestamps of machines with up-to-date leapsecond tables.
I disagree with your disagreement. I ran machines using ado 'right_only'
mode and synchronized by NTP, and they sailed smoothly through leaps with
no fudging. The NTP code was modified to ignore leap warnings, and to add
a single call to time2posix(). If those machines still existed they would
still be synchronized no matter whether their leapsecond tables were up-
to-date or not (although they would hiccup over leaps they were unaware
Part of the problem might be in what you refer to as "timestamps". An
ado 'right_only' system certainly doesn't believe that its raw time_t is
a form useful as an external representation.
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