I suspect the database that you use to load your gridview have a datatype datetime for time field .If so,get only part from the time field in gridview that you want in your excel.
<asp:BoundField DataField="date1" HeaderText="Date Of Leave" SortExpression="date"
DataFormatString="{0:dd/MM/yy}" />
DataFormatString examples:
{0:dd MMMM yyyy} - gives 24 February 2006
{0:MMM dd}-gives Feb 24 (substitue MMM with MMMM for the full month name instead of abbreviation)
{0:dd/MM/yy}-gives 24/02/06
{0:dd/MM/yyyy}-gives 24/02/2006
You can have look at
this link for further reference on how to use dataformatstrings.