Why datepicker returning null?

I have two textboxes the start_date and end_date . My problem is when I click the search button it gives a null value. I want to alert and display the value inside the textbox. I think the problem begins is when I clicking the toggle button. I’m using jquery with Vuejs. Can somebody help me with my problem? Here’s my jsFiddle -> https://jsfiddle.net/vL4z071a/