1-6.9 Date precision.
There are two senses in which it is important to specify precision.
One sense was referred to above. It has to the Gregorian calendar, which allows specification of day,
month, or year as the smallest known unit, is related to the semantic interpretation of a date as a
period of time. The other is in the sense of a prefix describing its general nature, such as "about,"
"calculated," "estimated," "before," "after," "probably before," etc. The rule specifies the type of
event and prefix of the input date (of P1) and the prefix of the output (propagated) date belonging
to the event of P2 being assumed.
