How to display subtraction of DateTime MySql in GridView

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C#

MySQL

Gridview

DateTime

I've stored the expiry date in MySQL DB and so I just wanted to display the date that's subtracted to Now() datetime in GridView.

I've seen a lot of answers regarding the subtraction of DateTime but the problem is, they all use Behind Code (C#). I don't know what to do after coding behind.

This is what I did to the GridView page...

<asp:TemplateField HeaderStyle-CssClass=" panel-heading text-center" HeaderText="DUE">
                    <ItemTemplate>
                        <asp:Label ID="countdown" CssClass="col-sm-10 text-center" runat="server" Text='<%# String.Format("{0} days left", Eval("countdown", "{0: dd/MM/yyyy}")) %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>

Is there any way to display the subtracted DateTime value in GridView?
My idea is like this...

        //get countdown from db               
                        DateTime countdown_new = new DateTime();
countdown_new = dataReader.GetDateTime(dataReader.GetOrdinal("countdown")); 
                        DateTime now = DateTime.Now;
                        int countdown = (int)(countdown_new - now).TotalDays;

//to be displayed in GridView
 string display = "Time left:" + countdown.Day + "Days";

I don't know what else to do...
So if possible, is there any way to just use the String.Format Text in GridView Label?