replaced .datepicker('update'); to .datepicker('refresh');

Changing value in 'input.date.start' was throwing "Uncaught TypeError: Cannot call method 'css' of undefined" upon calling .datepicker('update').

Looking at datepicker docu, I can't see reference to a 'update' method, but there is 'refresh'. Changing to 'refresh' solves the problem.

http://api.jqueryui.com/datepicker/#method-refresh