Time Zone File format - When does tzh_ttisgmtcnt or tzh_ttisstdcnt differ from tzh_typecnt, and how, and why?

[Olson, Arthur David (NIH/NCI) [E]]

This is indeed historical; the tzh_tisgmtcnt and tzh_ttisstdcnt header fields (and associated variables) were introduced in 1995 to allow European time zones to be used as time zone templates, and the software had to be set up to handle older files from which the information was absent (and in which the header fields were zero).

				--ado

From: Jonathan Leffler [mailto:jonathan.leffler at gmail.com] 
Sent: Monday, December 10, 2007 1:20
To: Time Zone Mailing List
Subject: Time Zone File format - When does tzh_ttisgmtcnt or tzh_ttisstdcnt differ from tzh_typecnt, and how, and why?


In tzfile.5.txt, it says:

tzh_ttisgmtcnt
    The number of UTC/local indicators stored in the file.

tzh_ttisstdcnt
   The number of standard/wall indicators stored in the file.
...
tzh_typecnt 
    The number of "local time types" for which data is stored in the file (must not be zero).

Empirically, in the tzdata2007h and tzdataj data sets, the values of tzh_ttisgmtcnt, tzh_ttisstdcnt and tzh_typecnt are always identical. 

Under what circumstances are there any differences in the three values?

If the answer is "when the stdcnt or gmtcnt is zero", when does that happen?
If the answer is "when the stdcnt or gmtcnt is a non-zero value different from typecnt", then what do the values mean? 

If the answer is "there isn't any difference any more, but historically, there was a transition period when they weren't all present and the stdcnt and gmtcnt were sometimes zero back then", then I can live with that as a satisfactory enough explanation. 


-- 
Jonathan Leffler <jonathan.leffler at gmail.com>  #include <disclaimer.h>
Guardian of DBD::Informix - v2007.0914 - http://dbi.perl.org
"Blessed are we who can laugh at ourselves, for we shall never cease to be amused."