Handling of dates for text

[cwittern]

Here is another idea for handling the dates in texts.
We will use the Julian day number to identify dates.
The base for the notation will be formed like this: JD+7digits+indicator
This is used to form identifiers that can be used to mark a date, which link to a taxonomy of dates.
The indicators are capital letters for time periods of 1 year and above
A : first day of a dynasty -- we will have a table for this -- do we also need this for reigns? or we use the Roman numeral closest?
C: first day of a century and so forth, as in the roman numerals:
I V X L C D M
1 5 10 50 100 500 1000
shorter periods are lowercase letters:
m : first day of month
d: single day
s: first day of sixty day period
t: first day of ten day period
This allows us to define a time period with one single code
For sorting purposes, we might actually use a day from the middle of the period in question.

In the texts, @jd attributes will carry such a JD value, preceded by # to indicate a reference

This should in general be on a grouping element above the seg level, depending on the range that has to be used, for most texts it will be the outermost element -- to avoid issues with content of the teiHeader.