Timezone | Gauquelin5

In g5 database, "legal time" means "the time that can be read on the birth certificate".
Abbreviations used in this page:

LT	= legal time
TZO	= timezone offset
UT	= universal time

Sources of information

Abbreviations for information sources:

OL	Olson database
FG	"Problèmes de l'heure résolus pour le monde entier" (by Françoise Gauquelin, Guy Trédaniel éditeur), 1991
IA	"The International Atlas: World Longitudes & Latitudes Time Changes and Time Zones" (by Thomas G Shanks, ACS Editions), 5th edition, 1999

Olson database

The IANA Time Zone Database, also called the Olson database, is the reference timezone data used by most programming languages.
It can be downloaded from www.iana.org/time-zones ; the last version of the code can also be downloaded from Paul Eggert's github on github.com/eggert/tz.
In the complete distribution, a file called theory.html permits to know that the Olson database is designed to provide precise information for dates after the POSIX Epoch (1970-01-01 00:00:00 UTC). It also provide timezone information for previous dates, but doesn't handle particular cases, for example war periods when one country is split in different timezone regimes.

Format used in g5

In g5 database, dates are stored using 2 distinct fields :

One field to store date and time if present.
It respects ISO 8601 format and can be one of :
- YYYY-MM-DD HH:MM:SS if time is known with precision of a second - ex: 1982-08-23 23:31:20
- YYYY-MM-DD HH:MM if time is known with precision of a minute - ex: 1928-11-13 22:15
- YYYY-MM-DD if time is not known - ex: 1928-03-28
One field to store timezone offset.
The format can be :
- sHH:MM when precision of a minute is sufficient - ex: +01:00 or -01:00
- sHH:MM:SS when precision of a second is needed - ex: +00:23:15 or -08:12:44
(s stands for "sign", plus or minus)

Expressing timezone offset

Using the vocabulary of CSV files generated from the database,

DATE-UT + TZO = DATE

A surprising feature of historical data is that historical notation does not respect the ISO 8601 definition of timezone offset.
The opposite value is always indicated.
This can be observed in Arno Müller files, in Cura web site and in both books FG and THM.

For example, for Brigitte Bardot, born 1934-09-28, Müller and both books indicate an offset of -1h.
Using the ISO 8601 definition,the offset for this date is +01:00

G5 program uses ISO 8601 definition

Meaning for historical usage
`LT + TZO = UT` If LT is known, add the TZO to obtain UT
Meaning for ISO 8601 and g5
`UT + TZO = LT` if UT is known, add TZO to obtain LT This is equivalent to `UT = LT - TZO` If LT is known, substract the TZO to obtain UT

Example : the first record of file D10 : Aaron Harold, born 1921-06-21 07:00
Cura gives TZO = 6h
g5 gives TZO = -06:00

To find UT :
From Cura value : UT = 07:00 + 6h = 13:00
From g5 value : UT = 07:00 - (-6h) = 13:00

OK, the results are equivalent.
This is not a blocking problem as it's finally just a matter of convention.

G5 integration

Time zone computation is a weak point of current implementation :

Edge cases (like end of WW2) are not computed.
When 2 sources contradict, each case should be solved in a documented way.

As a consequence, offset computation in g5 is partial and should be a priori distrusted.
Persons have 3 fields to express a date and its timezone :
- birth.date : the legal time
- birth.tzo : the timezone offset
- birth.date-ut : universal time
With (see above) : date-ut + tzo = date

Examples of contradictions between sources

For Italy, both books say that offset is 1h for the whole country since 1893-11-01.
Olson database says 1901-12-13
For France or Italy, FG says that "local time" is local mean time.
THM talks about local real time.
The difference between real and mean local time is the equation of time.

For such differences, the choice was made to use FG book.