DTV_ — ISO 80000 & Date/Time Foundation Vocabulary

The Date-Time Vocabulary (DTV) document FTF beta 1 draft Mike sent on Dec. 8, 2011 shows a concept of 'time unit' specialized as:

• 'precise time unit', a specialization of 'measurement unit'
• 'nominal time unit'

It is understood that in DTV, a 'precise time unit' is a kind of ISO 80000 measurement unit (i.e., 3.8 in ISO-80000-1):

real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can
be compared to express the ratio of the second quantity to the first one as a number

The DTV names for 'time unit' and 'nominal time unit' are misleading because in the Dec. 2011 DTV FTF 1 beta document,
these terms are not defined as kinds of ISO 80000 measurement units.

From the DTV FTF perspective, the objection against defining 'time unit' and 'nominal time unit' as ISO 80000 measurement units is that 'month',
a kind of 'nominal time unit', varies between 28 and 31 days and therefore does not exactly fit the definition of measurement unit per ISO 80000.
In pure ISO 80000 terms, this would suggest that 'month' would be a measurement unit with a variable conversion factor from 'day', which is defined as a normative measurement unit in ISO 80000-3, item 3-7.d.

I believe that the DTV interpretation of ISO 80000 measurement unit is too restrictive and should be changed such that a DTV 'time unit' is in fact a kind of ISO 80000 'measurement unit'.

There is compelling evidence to support this change in ISO 80000 itself:

1) The definition of year as a non-SI measurement unit in ISO 80000-3, Annex C, item 3-7 shows an example where the conversion factor is variable:

a := 365d or 366d

One tropical year is the duration between two
successive passages of the Sun through the mean
vernal equinox.

This duration is related to the corresponding difference
in mean longitude of the Sun, which depends on time in
a not exactly linear form; i.e. the tropical year is not
constant but decreases at a rate of nearly per
century. The tropical year is approximately equal to
365,242 20 d ≈ 31 556 926 s.

2) The value of a quantity can be expressed in three ways according to ISO 80000-1, item 3.19:

• a product of a number and a measurement unit
• a number and a reference to a measurement procedure
• a number and a reference material

3) 'month' could be defined as an ISO 80000 'conventional reference scale', that is, a quantity-value scale defined by formal agreement.

This could facilitate defining that 'month' is a 'conventional reference scale' varying between 28 and 31 'days'.
Specializations of 'month' could be made for 28-days months, 29-days months, 30 days months, 31 days months where such specializations can be defined as ISO 80000 derived units of 'days' with a precise conversion factor.

4) 'amount of substance', one of the 7 base quantities in the International System of Quantities, ISQ, is intrinsically context-dependent:
See the remarks in ISO-80000-9, 9-1:

Amount of substance of a pure
sample is that quantity that can
often be determined by
measuring its mass and
dividing by the molar mass of
the sample.

Amount of substance is
defined to be proportional to
the number of specified
elementary entities in a
sample, the proportionality
constant being a universal
constant which is the same for
all samples.

The name “number of moles”
is often used for “amount of
substance”, but this is
deprecated because the name
of a quantity should be
distinguished from the name of
the unit.

In the name “amount of
substance”, the words “of
substance” could, for
simplicity, be replaced by
words to specify the substance
concerned in any particular
application, so that one may,
for example, talk of “amount of
hydrogen chloride, HCl”, or
“amount of benzene, C6H6”.
It is important to always give a
precise specification of the
entity involved (as emphasized
in the second sentence of the
definition of the mole); this
should preferably be done by
giving the molecular chemical
formula of the material
involved.

Just like 'amount of hydrogen chloride' is a specialization of 'amount of substance',
'January' is a specialization of 'month.
All are measurement units in the sense of ISO 80000.

The DTV specification should clearly indicate the correspondence between the DTV vocabulary and the corresponding ISO 80000 vocabulary.
These correspondences are important to clarify the relationship between the use of ISO 80000 vocabulary in DTV and SysML's QUDV.

The FTF consulted with NIST authors of VIM and ISO 80000-3, who confirmed that 'month' and 'year' are not defined by SI and do not fit the VIM definition of 'measurement unit'. Therefore, the proposal made above is not adopted. Instead, a Rationale section is added to discuss the issue.