How to validate date type in POST payload with flask restplus?

Consider the following:

from flask import Flask
from flask_restplus import Api, Resource, fields

app = Flask(__name__)
api = Api(app)
ns = api.namespace('ns')

payload = api.model('Payload', {
    'a_str': fields.String(required=True),
    'a_date': fields.Date(required=True)
})

@ns.route('/')
class AResource(Resource):

    @ns.expect(payload)
    def post(self):
        pass

If I POST {"a_str": 0, "a_date": "2000-01-01"} I get 400 as expected, because a_str is not a string. However, when I POST {"a_str": "str", "a_date": "asd"} I don't get 400. Here I would like to also get 400, because "asd" is not a common date format.

I looked into the Date class doc and I see that there is a format and a parse method which should check whether the string is in a common date format. However, they do not seem to be called here.

Is there another way how to do this? Currently I am validating the date format by hand, but it seems that fask restplus should be able to do it for me.

As @andilabs mentions, it really weird to define expected payload twice. You can define expected payload by using only RequestParser as so:

from flask import Flask, jsonify
from flask_restplus import Api, Resource, fields, reqparse, inputs

app = Flask(__name__)
api = Api(app)
ns = api.namespace('ns')

parser = reqparse.RequestParser()
parser.add_argument('a_str', type=str)
parser.add_argument('a_date', type=inputs.datetime_from_iso8601, required=True)


@ns.route('/')
class AResource(Resource):

    @ns.expect(parser)
    def get(self):

        try:  # Will raise an error if date can't be parsed.
            args = parser.parse_args()  # type `dict`
            return jsonify(args)
        except:  # `a_date` wasn't provided or it failed to parse arguments.
            return {}, 400


if __name__ == '__main__':
    app.run(debug=True)

Test by using curl:

$ curl -XGET -H "Content-type: application/json" -d '{"a_str": "Name", "a_date": "2012-01-01"}' 'http://127.0.0.1:5000/ns/'
{
  "a_date": "Sun, 01 Jan 2012 00:00:00 GMT", 
  "a_str": "Name"
}