How can I calculate age in years and days

Question

How do I calculate the age in years and days i.e I am 25 year and 100 days old.

I am getting only year, how can I find days too?

XML View:

<DatePicker id="BirthDate" placeholder="Select Date ..."
                change="StarthandleChange" width="50%" class="sapUiSmallMarginEnd"/>
<Button text="click" press="OnPressShowAge" class="sapUiSmallMarginEnd"/>

OnPressShowAge: function(){
     var BirthDate = this.getView().byId("BirthDate").getDateValue();
     var year = BirthDate.getFullYear();
     var today = new Date();
     var age = today.getFullYear() - year;
     alert(age);
}