Proper way to save date input data to Firebase with simpleLogin

I'm new to firebase, and I'm trying to add a Date Of Birth field to a form and save it to Firebase.

The input field in the form looks like this:

<label for="birthdate">Date of Birth</label>
  <input type="date" id="birthdate" name="birthdate" 
  ng-model="user.birthdate" ng-required="true" placeholder="Last Name">
  <p class="error validationerror"
  ng-show="myform.birthdate.$invalid && myform.birthdate.$touched">
  Birthdate is required</p>  

Following a lynda.com tutorial, a working register function was created like this:

register : function(user) {
      return simpleLogin.$createUser(user.email, user.password)
      .then(function(regUser){
        var ref = new Firebase(FIREBASE_URL + 'users');
        var firebaseUsers = $firebase(ref);

        var userInfo = {
          date: Firebase.ServerValue.TIMESTAMP,
          regUser: regUser.uid,
          firstname: user.firstname,
          lastname: user.lastname,
          email: user.email
        }

        firebaseUsers.$set(regUser.uid, userInfo);
      }); //add user
    }, //register

I tried to simply add another birthdate: user.birthdate key:value pair to the end after email: user.email but that didn't work.

register : function(user) {
      return simpleLogin.$createUser(user.email, user.password)
      .then(function(regUser){
        var ref = new Firebase(FIREBASE_URL + 'users');
        var firebaseUsers = $firebase(ref);

        var userInfo = {
          date: Firebase.ServerValue.TIMESTAMP,
          regUser: regUser.uid,
          firstname: user.firstname,
          lastname: user.lastname,
          email: user.email,
          birthdate: user.birthdate
        }

        firebaseUsers.$set(regUser.uid, userInfo);
      }); //add user
    }, //register

Nothing gets added to firebase for the birthdate.

If I do console.log("user.birthdate: "+ user.birthdate); right after var userInfo I can see the date that was chosen in the date picker... so I know its getting from the form into that register function.

I'm guessing that simpleLogin can only have firstname, lastname, and email?