[tz] Dealing with Pre-1970 Data

Guy Harris guy at alum.mit.edu
Sun Sep 1 23:20:39 UTC 2013

On Sep 1, 2013, at 3:35 PM, Lester Caine <lester at lsces.co.uk> wrote:

> The reason I've been 'hammering' the LMT aspect is because we know that at some point that will become prevalent,

Presumably by "at some point" you mean "at some point in the past".

The problem is that if you're dealing with times far enough back that you need to worry about local mean solar time, you might need to worry about local mean solar time at some particular location, in which case you don't have a tzid for a zone, you have a longitude value, and can just plug that into the formula.

> but I am quite happy that when the tz returned is 'LMT'

Presumably by "tz" you mean "time zone abbreviation", i.e. if somebody looks up a time in the America/New_York zone prior to 1883-11-18 17:00:00 GMT (well, UTC, really), that will be a case where the tz returned LMT?

> then a 'local time offset' is required - based on longitude rather than a fixed 'zone' value.

...which means that you won't get, for example, the right conversion for times if the longitude is sufficiently far from whatever longitude was used to generate the "LMT" entry for that zone.

So if "LMT" is the time zone abbreviation for a given date/time value, whoever's doing the conversion needs to do their own calculation based on the longitude, rather than using any of the data from the tzdb.

Is that what you're saying?

> This would pre-date an 'LMTZ' offset where the zones of common time started to appear?

At least for the US, if "'LMTZ' offset" refers to the offsets for zones given as local time from a certain specified meridian in the 1918 Standard Time Act (and perhaps also in whatever railroad rules and, perhaps, US state laws), that's the case.

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