Today I was struggling to pass the date value from jquery UI date picker to code behind in C# so I know when the someone would like to get the task done (due date). The problem was, none of the suggestion worked. Jquery was misbehaving when setting the value of the hidden text. [SOLVED]: The solution was to define a name of the hidden field instead of referencing using an id which did not work. So I defined a name= 'nameHere' and then after the user clicks on the date, in code behind, I just reference the name of the hidden field like the way you would get a session id but in a form request.
String DateText = Page.Request.Form["DateValue"];