Add a simple data type for dates

[mbrush]

Can extend string, like iri and code data types do.

Consider if we want date or dateTime.

See proposals from Matt's and Bob's respective VA- and GA4GH- data type documents.

[larrybabb]

in json schema we are using the format attribute that is a sibling to the type: string declaration that allows us to control whether the particular property is a specific format of a date (time, date or datetime).

I suppose we could add something to the metaschema processor to maybe make the various data datatypes appear more cleanly, but I think we should defer that until there is a more urgent need.

If this is primarily about the spec and it's documentation we can develop a way to generate these date-based datatypes in the documentation.

[larrybabb]

@ahwagner What say you about adding two simple/primitive datatypes in the spirit of IRI and code called date and datetime that simply predefine the string format so that it is more transparently represented in our documentation and consistently applied in our models?

[ahwagner]

It's a good question and has me rethinking the decision around IRI; it might be simpler to just use the built-in type and format options. Same for date and datetime.

code is useful since it has a regex associated with it that I would only want to update in one place.