[tz] Anecdotal discrepancy for Navajo nation in Arizona

Guy Harris guy at alum.mit.edu
Thu May 26 08:17:55 UTC 2016

> On May 24, 2016, at 10:11 PM, Deborah Goldsmith <goldsmit at apple.com> wrote:
>> How do OS X and iOS deal with local time in AZ/Navajo/Hopi/Page,
>> and federal parks and facilities  in AZ?
> I gave the one data point I have. :-)

Which was

>> His phone must have been associated with a cell tower inside the Navajo nation, because his zone was set by NITZ to America/Denver (DST in effect).

OK, so I've read 3GPP TS 22.042 version 13.0.0 Release 13, and it says:

	The serving PLMN shall make Local Time Zone (LTZ) available to the MS as an offset from Universal Time in units of 15 minutes.

	When the LTZ is compensated for DST (summertime), the serving PLMN shall provide a DST parameter to indicate this. The adjustment for DST can be +1h or +2h.

which looks like it corresponds only to the information in one of the entries for an IANA tzdb zone (but without the time zone abbreviation), not to all of the entire tzdb zone's (i.e., it's useless for converting any times before or after any transition times).


	1) Is it known that it was set by NITZ, or is it just assumed that it was set by NITZ rather than "take the location given by Location Services, look it up in the shape maps, and convert that to a tzdb tzid"?

	2) If so, how does one determine a tzdb tzid, such as American/Denver, from the offset from UTC in units of 15 minutes and "a DST parameter" in whatever form it takes (presumably some *other* 3GPP spec says how that stuff goes over the air, because TS 22.042 v13.0.0 sure doesn't), as would be required by the zone "[being] set by NITZ to American/Denver"?

If the system time zone *wasn't* set by NITZ, then, in answer to Brian Inglis's question:

	1) it's presumably handled by code that uses tzdb files (definitely for UN*X APIs, although I think some frameworks have their own code to read the tzdb files - I seem to remember that the two separate projects containing the two bits of code had their own copies of the tzdb source files and compiled them independently, but that the tzdb files were later put into a separate "tz data" project, so the two bits of code would at least share the underlying data);

	2) OS X 10.11.5's tzdb files look as if they have the usual America/xxx zones;

	3) as far as I know, somewhere in the bowels of OS X and iOS there's code that can get information from Core Location:



	   look it up in what I presume are maps of the tzdb zone boundaries, and determine the appropriate tzdb tzid, which is the behavior if you tell the machine to automatically pick the zone - you can also manually configure it.

So, by "However, he observed that the parts of the Navajo nation adjacent to Page were actually observing the same time zone as Page (America/Phoenix, DST not in effect).", do you mean "if he went to parts of the Navajo nation adjacent to Page, his phone reported DST not being in effect" or "if he went to parts of the Navajo nation adjacent to Page, the clocks he saw reported DST not being in effect"?

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