Instead of getting control with dynamic name.Simply make it ClientIdMode="Static" in your textbox control.this will fix your problem..Beware of this will create some ajax issues.

  you are using inside Update panel please paste Design code ...

Sample check
The error is here:

    var fdate = document.getElementById('txtFromDate');
    var edate = document.getElementById('txtToDate');

The problem is that txtFromDate and txtToDate are the server name of controls, not the client name (look the source of page from the browser).

Try this:

    var fdate = document.getElementById('<%=txtFromDate.ClientID%>');
    var edate = document.getElementById('<%=txtToDate.ClientID%>');

getting error while calling function from ajax enabled content page.

 Post your Javascript function please