if (Convert.ToDateTime(grid.Rows[e.RowIndex].Cells["date_done"].Value) > new DateTime(1900, 1, 1))
{
    e.CellStyle.Font = new System.Drawing.Font("Microsoft Sans Serif", 8.25F, FontStyle.Regular, GraphicsUnit.Point, ((byte)(0)));
    e.CellStyle.ForeColor = Color.DarkGreen;
}