[LEAPSECS] Footnote about CCITT and UTC

Zefram zefram at fysh.org
Fri Dec 12 17:24:09 EST 2008

Poul-Henning Kamp wrote:

>PS: According to Wikipedia, ISO 31-1 defines a day as 86400 seconds,

>anybody here who can verify if this is really the original text ?

Yes. ISO 31-1:1992(E) lists the following as units of time "which may
be used together with SI units because of their practical importance or
because of their use in specialized fields":

minute min 1 min = 60 s
hour h 1 h = 60 min = 3 600 s
day d 1 d = 24 h = 86 400 s

This is of course defining a *unit of measurement* called "day", not
a calendar unit. Likewise the hour and minute. To be such, they are
necessarily constant quantities. Calendar units have no such inherent

I understand that astronomers similarly define a unit of time called
"year", symbol "a", with value exactly 365.25 d = 31 557 600 s. (This is
based on the mean Julian year.) This unit is not mentioned in ISO 31-1.
The standard does mention the tropical year, as a unit of no status
(listed for information only), but of course that's a different quantity
with no such exact definition. It also lists (for information only)
the light year, giving an approximate value that is sufficiently precise
to show that it is based on the 365.25 d year rather than any other year.

There is a problem with the symbol "d" for day: it means that the
centiday (a very useful unit) clashes with the candela for the symbol
"cd". SI itself avoids such clashes, and I'm slightly surprised to see
ISO endorse these symbols without comment.


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