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A general description is here.
Where time zones have changed abbreviations over the years, the software tries to more consistently use the current abbreviation. On some platforms where the C function ‘localtime’ changed the ‘tzname’ variables the reported abbreviation could have been that of the last time converted.
The code for date-times before 1902 assumed that the offset from GMT in 1902 was a whole number of minutes: that was not true of Paris (as recorded on some platforms).