<lightning:input
        aura:id="field"
        type="datetime"
        name="datetimeInput"
        label="Set up a date/time"
        value="{! v.scheduleDatetime }"
        required="true"
        onchange="{! c.onChange }"
    />

</aura:component>

onChange: function(component, event, helper) {
    console.log("onChange", event.getSource().get("v.value"));
    const target = event.getSource();
}

onChange 2019-05-28T22:30:00.000Z