DateTime field

I have a question. Why SharePoint datetime field display today date although the item column it empty? How to solve this problem. Here is my code

if (cItem != null && cItem["DateSitePickup"] != null)
        dtSitePickupDate.SelectedDate = (DateTime)cItem["DateSitePickup"];

The item column "cItem["DateSitePickup"]" is empty. It seems do not run this code. But the control "dtSitePickupDate" show today's date (change every day).

Please help me out. Many thanks.