Addition: I encountered problem when continiously use
dpn.select(dt); dpn.updateFreeBusyDays() // or dpn.visibleRangeChangeCallBack()
you will sometimes receive error __pendingCallbacks[i] is null.
This is a .net bug (see http://www.codeproject.com/KB/aspnet/pendingcallbacks.aspx). So i fyou use a "i" variable for inside loops in "select", "updateBusyDays", "visibleRangleChangeCallback" please use another variable :)