finish date() spec

The date spec is currently unfinished. It says:

```Converts to a string in the ….date format.```

I'd like to propose to add a time and local timezone offset (to the ODK spec), i.e. to treat it as a dateTime. This seems to fall within the possibilities of the spec for "date" that we refer to in the ODK spec: https://www.w3.org/TR/xmlschema-2/#date

So `date(12345.5)` would be turned into e.g. `2003-10-20T08:00:00.000-04:00`.

One use case that has come up is:

a. converting a date with `decimal-date-time()`
b. do some stuff with the result of a, e.g. add 1 (day)
c. convert back the result of b using `date()` and get a predicable result (e.g. 1 day later).

Start with the unfinished date() wording in the ODK specification and read the referenced W3C XML Schema date definition. Resolve whether date() should include a time and local timezone offset, define the expected decimal-date-time conversion behavior, and update the specification with an unambiguous example.

Written by the indexing model from the issue text.