[tz] Dealing with Pre-1970 Data

Lester Caine lester at lsces.co.uk
Sun Sep 1 19:27:58 UTC 2013

Guy Harris wrote:
> On Sep 1, 2013, at 6:40 AM, Lester Caine<lester at lsces.co.uk>  wrote:
>> >Zefram wrote:
>>>> >>>Is the half hour rounded up or down.
>>> >>You need to make an arbitrary decision, and sure, for some applications
>>> >>you need to make sure that all parties make that decision the same way.
>>> >>But that's totally out of scope for the tz database; it's not our place
>>> >>to specify a canonical choice of rounding mode.
>> >
>> >The database - yes - but we are talking about the backup material that is used when the database fails. Pre-1970 for example;)
> What do you mean by "when the database fails"?  You presumably don't mean "when the database contains no data", as that is*NOT*  uniformly true of pre-1970 times.  "The database" is the text files, in zic format, in the tzdata collections, so the database*does*  contain data for pre-1970 times; however, we make weaker claims about its accuracy and completeness.
> One thing we definitely do not do, and should not do, in the tzcode reference implementation is map all times prior to 1970 to LMT, so it's not as if "pre-1970" means "LMT".
> If you mean "when the time being converted is prior to the introduction of standardized time", the only thing we provide is the first "Zone" line, which currently has what is, I guess, an LMT value for some location within the city used in the tzid for the zone in question - and that's "LMT routed to a*one-second*  boundary", e.g.:
> 	Zone America/New_York -4:56:02 - LMT 1883 Nov 18 12:03:58
> As far as*I'm*  concerned, anything having to do with non-standardized time, such as LMT and local apparent time, is, and should always be, out of scope.  People who need LMT, or local apparent time, can, and must, calculate it themselves.

My problem with that statement is ensuring that the 'calculations' match with 
those used to create the values IN the database. 'Guessing' how the initial time 
was generated is not an acceptable way of going, since you can't then compare 
accurately between a 'guessed' time and a calculated one. As a minimum where a 
value has come from needs to be documented, and essentially that uses the 
equation I'm looking for, and a defined location. Following on from that, 
indicating that some amendment from this base is in place and can be trusted 
would be useful. I'm more than happy that the data returned for the UK is all 
correct, and I can adjust the Isle of Man record if I need to, but how do I 
assess the accuracy of other historic data? The only think I am told is correct 
is post 1970 information.

Lester Caine - G8HFL
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