After getting a document via mongoc_collection_find_with_opts, how can one retrieve its object values. In all the documentation and tutorial examples, the only thing seems to make a JSON representation of the document. For instance, getting a document from the restaurants collection (with a resctricting projection):

{"borough":"Bronx","cuisine":"Bakery","grades":[{"date":{"$date":1393804800000}},...],"name":"Morris Park Bake Shop"}

In particular, unlike when exporting a collection, dates are numerical and there are no examples explaining how to retrieve a human readible value.
When accessing MongoDB from java, there are a set of functions enabling to retrieve values from a document, such as getDouble, but looking in C driver API reference, there are no functions related to bson_document. I am missing something or are they missing from the C driver API?
If so, the interest of this driver is very limited.