The reason that 'Names, Dates, People, and Places' are all one chapter is one of historical legacy. The names/people/places sections have so developed as to swamp the minor discussion of dates.
I propose that we:
a) split off Dates http://www.tei-c.org/release/doc/tei-p5-doc/en/html/ND.html#NDDATE to its own separate small chapter.
b) move the creation of att.datable.custom att.datable.iso att.duration.iso there instead of ND
c) rename the chapter 'NP' instead of 'ND'
d) reference the new chapter from http://www.tei-c.org/release/doc/tei-p5-doc/en/html/CO.html#CONADA
e) expand new chapter to give more examples of @calendar and dating edge-cases
f) consider merging timeline information into new chapter and/or referencing from chapter what is at http://www.tei-c.org/release/doc/tei-p5-doc/en/html/SA.html#SASYMP
I recognise there are many reasons for not doing this (persistent URLs, etc.) But I think separating out date/time related markup into one place might make more sense. Arguably it might make more sense to undertake this simultaneously to doing the same kind of things with sections in other chapters.
Just thinking out loud.