This is used for `YYYY-mm-DD` type-dates. They are not necessarily date-times, but anything which can be instantiated as a javascript date: http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.1.15

The problem with `format: 'date-time'` is that you might not want to pass `YYYY-mm-DDTHH:MM:SS+07:00` formatted strings into your application- `date-time` requires such precision whereas this regex allows the more friendly `YYYY-mm-DD` format.

Hmm- we should probably test it. Honestly we should have a more stringent & complete regex that corresponds to any date instantiable in javascript, but then you can't read in other 'hand-created' json-schemas that someone may have created... perhaps that is not a valid use-case, however.