Isn't this how you simulate a Grails datepicker? (Grails 2.4.2)

PersonController.groovy

package g242

import grails.converters.JSON

class PersonController {

    def testPost(Person person){
        log.debug "JSONning ${person}"
        render person as JSON
    }
}

PersonControllerIntSpec.groovy

package g242

import spock.lang.*


class PersonControllerIntSpec extends Specification {

    PersonController controller

    def setup(){
        controller =  new PersonController()
    }

    void "posting dates to testPost()"() {
        given:
        controller.params.name = "Johnny"
        controller.params.birthday = "date.struct"
        controller.params.birthday_day = 3
        controller.params.birthday_month = 6
        controller.params.birthday_year = 1980
        controller.request.method = 'POST'

        when:
        controller.testPost()

        then:
        controller.response.json.birthday
        
        /* Fails with:
Condition not satisfied:

controller.response.json.birthday
|          |        |    |
|          |        |    null
|          |        [id:null, birthday:null, name:Johnny, class:g242.Person]
|          org.codehaus.groovy.grails.plugins.testing.GrailsMockHttpServletResponse@9b05ade
g242.PersonController@40bc4a46
        */
        
    }
}

Changing controller.params.birthday = "date.struct" to controller.params.birthday = "struct" fixes it.