1-6.4 Elements of a semantic base for dates.
The semantic interpretation of a date is
determined to be two Julian day numbers, one specifying the beginning of a time period, and the
other the end. There must also be some indication of the precision of each of these real numbers.
Sometimes this number is known to be accurate only to the year (365.0, i.e., ±182.5) and sometimes
to the day (1.0, i.e., ±0.5). Even with precision this representation with just two day numbers should
be extended. Approximations are more than the modification of the precision of the beginning and
end of a time period. Normally there is a middle point between the two ends that we are fairly
confident about. Usually the plus and minus tails (of a skewed probability distribution) are not
symmetric. Thus we need to specify three, not two time periods. Sometimes also the two ends are
a terminus post quem and a terminus ante quo, i.e., we are fairly confident only that the event took
place at some time between the two dates.
