This piece of code will give you the date in the format you are looking for.
string myDate = DateTime.Now.ToString("yyyy/MM/dd")
The best place to convert the date to the format you want will be in the method where you convert List to Datatable, so you don't have to do all these when the data is bound to the datagrid.
For this you have to modify the converter method, where you identify the property by name and change the format.
Hope this helps.