ISO
8601:2000(E)
6
©
ISO
2000
All
rights
reserved
¾
times
only;
¾
time-intervals;
¾
recurring
time-intervals;
¾
any
abbreviation
of
the
above.
4.3
Time-units
and
reference
systems
4.3.1
Time-units
Duration
referred
to
in
this
International
Standard
shall
be
expressed
in
one
or
more
of
the
following
units:
¾
second
:
a
basic
unit
of
measurement
of
time
in
the
International
system
of
units
(SI),
defined
in
ISO
31-1:1992
¾
minute
:
a
time-unit
of
60
seconds
¾
hour
:
a
time-unit
of
60
minutes
¾
day
:
a
time-unit
of
24
hours
¾
week
:
a
time-unit
of
seven
days.
¾
month
:
a
time-unit
of
28,
29,
30
or
31
days.
NOTE
In
certain
applications
a
month
is
regarded
as
a
unit
of
time
of
30
days.
¾
year
:
a
time-unit
of
12
months,
considered
to
approximate
the
duration
required
for
one
revolution
of
the
earth
around
the
sun.
See
also
4.3.2.1.
4.3.2
Date
and
time
reference
systems
4.3.2.1
The
Gregorian
calendar
This
International
Standard
uses
the
Gregorian
calendar
for
the
identification
of
calendar
days.
The
Gregorian
calendar
provides
a
reference
system
consisting
of
a,
potentially
infinite,
series
of
contiguous
calendar
years.
Consecutive
calendar
years
are
identified
by
sequentially
assigned
year
numbers.
A
reference
point
is
used
which
assigns
the
year
number
1875
to
the
calendar
year
in
which
the
"Convention
du
mètre"
was
signed
at
Paris.
The
Gregorian
calendar
distinguishes
common
years
with
a
duration
of
365
calendar
days
and
leap
years
with
a
duration
of
366
calendar
days.
A
leap
year
is
a
year
whose
year
number
is
divisible
by
four
an
integral
number
of
times.
However,
centennial
years
are
not
leap
years
unless
they
are
divisible
by
four
hundred
an
integral
number
of
times.
This
International
Standard
allows
the
identification
of
calendar
years
by
their
year
number
for
years
both
before
and
after
the
introduction
of
the
Gregorian
calendar.
For
the
determination
of
calendar
years
and
year
numbers
only
the
rules
mentioned
above
are
used.
For
the
purposes
of
this
International
Standard
these
rules
are
referred
to
as
the
Gregorian
calendar.
The
use
of
this
calendar
for
dates
preceding
the
introduction
of
the
Gregorian
calendar
(i.e.
before
1582)
should
only
be
done
by
agreement
of
the
partners
in
information
interchange.
NOTE
1
In
the
prolaptic
Gregorian
calendar
the
calendar
year
[0000]
is
a
leap
year.
NOTE
2
No
dates
shall
be
inserted
or
deleted
when
determining
dates
in
the
prolaptic
Gregorian
calendar
(this
may
be
necessary
for
the
calculation
of
dates
in
the
Julian
calendar
before
1582).
Also
note
that
the
year
numbers
of
years
before
the
