show duration between two times using javascript in

i have web application with user-control having two text boxes to which i have assigned timepicker property's through java script. i need to find duration between two time (between two text boxes) through java-script when any of the textbox value changes and set that duration value to label. Overall,My user-control contain two text-boxes and one label. However i have tried but not able display duration in label. This my javascript:---