Re: unix epoch data wrapping

[Jonathan T. <jt...@as...>]

On Sat, 27 Mar 2010, Ethan Merritt quoted from the gnuplot source code:
#              /* In line with the current UNIX98 specification by
#               * The Open Group and major Unix vendors,
#               * two-digit years 69-99 refer to the 20th century, and
#               * values in the range 00-68 refer to the 21st century.
#               */

On Saturday 27 March 2010, pl...@pi... wrote:
> OK, that is certainly the source of what is happening. That seems a 
> pretty arbitrary and dumb way to define behaviour that for no good 
> reason is based on the otherwise irrelevant unix year dot.

It's not based on the Unix time epoch -- the gnuplot %y-interpretation
switch is between 31 Dec 1968 and 1 Jan 1969, while the Unix time epoch
is a year later, on 1 Jan 1970.


> Still that's 
> the way it is and it's outside of gnuplot. I'm curious as to whether 
> this works that same on windows which thinks the world was created in 
> 1980, not 1970.

On Sat, 27 Mar 2010, Ethan Merritt wrote:
# Apparently MSWin (or at least Excel) uses a rule that
# 00-29 is a 21st century date, while  30-99 is a 20th century date.  
# I have no idea what they are expecting to happen in 2030.


If I write the 2-digit year "50", does it mean 1950 or 2050?
	[Let's leave aside the possibility of other centuries.]
In the original poster's case, I gather that 1950 would have been the
correct interpretation.  But if I were plotting future values of a
retirement portfolio, 2050 would probably be the better choice.
Fundamentally, there is *no* *way* for gnuplot to know the "correct"
choice, so it has to make an *arbitrary* choice (which will be wrong
a fair number of times).

IMHO gnuplot's current choice is a reasonable one (it's certainly the
only one I've ever seen in other software, albeit I've never used
MS-Excel).  So, IMHO this is a feature, not a bug.

But it would be useful to document this feature.  Here's a suggested
patch ('diff -u' format) to gnuplot.doc:

*** gnuplot.doc.orig	Sun May 17 22:47:35 2009
--- gnuplot.doc	Sat Mar 27 18:42:43 2010
***************
*** 9687,9696 ****
--- 9687,9700 ----
   it can still be printed with the "%a", "%A", "%b", or "%B" specifier:
   see `set format` for more details about these and other options for printing
   timedata.  (`gnuplot` will determine the proper month and weekday from the
   numerical values.)
  
+  In line with the current UNIX98 specification by The Open Group and major
+  Unix vendors, when reading two-digit years with %y, values 69-99 refer to
+  the 20th century, while values 00-68 refer to the 21st century.
+ 
   See also `set xdata` and `Time/date` for more information.
  
   Example:
         set timefmt "%d/%m/%Y\t%H:%M"
   tells `gnuplot` to read date and time separated by tab.  (But look closely at


-- 
-- "Jonathan Thornburg [remove -animal to reply]" <jt...@as...>
   Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
   "Washing one's hands of the conflict between the powerful and the
    powerless means to side with the powerful, not to be neutral."
                                      -- quote by Freire / poster by Oxfam