How to format a varchar in a gridview in military time to standard time

There is a varchar column in the database table in this format of military time 1230 or 1750 or 1320 and that field get populated into a gridview in asp. I need to convert that string to standard time. I have tried the DateFormatString and nothing seems to work.

the asp is

<asp:BoundColumn DataField="ScanTime" SortExpression="ScanTime" ReadOnly="True" 
      HeaderText="Scan Time" DataFormatString="{0:hh}:{0:mm} {tt}">
 <ItemStyle Width="80px"></ItemStyle>
 </asp:BoundColumn>
 

that DateFormatString causes Input string was not in a correct format. error.

Please help I have tried combinations of hh:mm and things like that but since its a varchar I think its failing

---