Hi,
As per my understanding you need to get the value of a date control which you have done using Javascript. On the post back you need to have this value in cs page and then get the value from SQL.
Solution
In this case please put a hidden field in the page.
<asp:hiddenfield id="hdValue" runat="server" xmlns:asp="#unknown" />
Then assign the value to the hidden field on selection using Javascript.
document.getElementById('hdValue').value = '<< Selected value from calender>>';
On post back you can get the value from the hidden field.
In case if you looking for data to be refreshed without postback then need to go for Call Back or Ajax.
Hope this helps.