We can’t directly pass the Date field value from Lightning Component to its Apex Controller. What we need is to:

1. Capture the value of Date in a String variable in Apex function’s parameter.
2. Convert that String value to Date value.
3. Use the Date value where we want to.

Example :

DateUsage.cmp

<aura:component>
    <ui:inputDate displayDatePicker="true" aura_id="expDate" format="MM-DD-YYYY"/>
</aura:component>

DateUsage.js

({
    createExpDate: function(component, event,helper) {
        var expirationDate = component.find("expDate").get("v.value");
        var action = component.get("c.createExpDate");
        action.setParams({ expDate : expirationDate });
        // callback and enqueue action
    }
})

DateUsageController.cls

public class DateUsageController {
    @AuraEnabled
    public static void createExpDate(String expDate) {
        Date expirationDate = Date.valueOf(expDate);
        // Now we can use expirationDate
    }
}

I hope this solution works for you to pass the Date field from Salesforce Lightning Component to Apex Controller.