Can't access combodate from JS function

I suspect that the problem I'm having is just a misunderstanding of JS scope or something.

I have moment.js and combodate.js linked and the following code works fine when run immediately after my form:

<script type="text/javascript">
    $(function(){
        $('#birthday').combodate();
    });
</script>

But if I want to run it in a function (to clear the form, for instance):

var launchChildRegisterModal = function() {
    //Do other stuff...
    $('#birthday').combodate();
}

I get the error:

TypeError: $(...).combodate is not a function

Do I have to do something special to use it from within a function?