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On Tue, 2 Aug 2011 10:08:40 -0400, Ray Denenberg wrote
> From: Edward C. Zimmermann
> > The same "problem" exists with date expressions. We have well defined
> > expressions of year, month, day, hour, minute, second,.. perhaps even
> > century precision.. but not decade, whence the generalized "x" syntax.
> 
> Ed - I appreciate your patient efforts to enlighten us on precision. I for
> one have been a bit slow to grasp your philosophy, particularly the role of
> the "x" syntax, but the above passage has provided some clarity and has
> motivated me to think this through more clearly (I hope).
> 
> So, If '1965' means "some date  - or point in time - during the year 1965",

Its a bit more philosophical.. er... ambiguous. A point can also be an 
interval.. and an interval at a coarser precision can be a point...
When, for example, was the WTC Tower 1 (aka North Tower) destroyed?
When did WW-II start? When did it end? When did the "Berlin Wall" fall? When 
did the "Cold War" start? Or.. how about the time of an explosion of a stick 
of TNT--- think about the velocity and wave?
All events have a start and an end.. BUT.. OK.. that is getting a bit too ...


> then '196x' means "some date -  or point in time - during the 1960s".
> 
> Right?

Its means a date (leave off "some" since its too loaded) in the 1960s. From 
the perspective of decade measurement precision its a point in time..

> 
> That's really not the way it is represented in the current draft. Rather, 
it
> is included within multiple dates. Based on my new (and hopefully correct)
> understanding I don't think it belongs either in multiple dates or one of a
> set.
> 
> But let's digress momentarily and revisit the 'u' syntax.  The draft spec
> currently says '199u' means "some unspecified year in the 1990s".   But do

While u can have a measurement semantics.. its more .. its both an explicit 
declartion of "unknown" or "uncertainty" AND a placeholder to one day get 
filled with the a known digit.

Example:  19u4-12-12 

This is saying that an event (measured in day precision) occured on the 12th 
of December in the 4th year of some, as yet unknown/unconfirmed, decade in 
the 20th century.

19uu-12-25
An event took place on one, as yet unknown, Christmas day in the 100 year 
range of 1900 to 1999.

When the "missing digit(s)" is one day disclosed there is little to indicate 
that the rest will change. We are in fact saying that it won't--- at least 
not from the perspective of the state of knowledge one had when one 
expressed the date.
With 'x' or the other precision expressions we are saying.. Yes.. it can 
change..  I wrote about knowledge and measurement a short while back...

A date expression 195x might become with a higher level of measurement 
precision 1960 or 1949.. 195u should not.. again.. think about weights.. a 
gramm balance that measures to 1 gram precision.. A sample can have 1 gram 
weight and yet with a lab balance that can measure to fractional mg it might 
have only 0.99001 grams.. It other words.. less than 1 gram.. Imagine timing 
events with a Sun dial, sand hour, cheap LCD watch, an atomic clock...


> we really want it to mean that or do we want it to mean: "some unspecified
> date - or point in time - during the 1990s"?
> 
> And if so, then wouldn't 199x mean the same thing as 199u?

It can.. But what would, by contrast, 19xx-12-25 mean? 
u is a placeholder.. x is not.. 

We can have 1u2u or even uu99 but not 1x2x or xx99. These, like, 19xx-12-25 
make no sense.  19xx is "100 year" precision.. 12-25 is day precision.. Its 
like saying.. I know how many rest micrograms a sample has but can only 
roughly estimate how many kilograms it has...  


From a programmer's prespective.. matching dates with u is simple globing.. 
like '?'. With x its not glob but comparing to least precision...

> 
> --Ray


--

Edward C. Zimmermann, NONMONOTONIC LAB
http://www.nonmonotonic.net
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