I use dnn calendar in my project as bellow:

llDate.NavigateUrl=DotNetNuke.Common.Utilities.Calendar.InvokePopupCal(txtEffectedDate);

when I click lnkCallDate, the calendar appears and i could select my date which displayed in txtEffectedDate. but in code behind, when i read txtEffectedDate.Text, it is empty (as if the selecteddate set to textbox just in client side) ?!! 

how can i read calendar selected date in code behind?!