//определение класса ImportFromExcel ... 
...
private Excel.Application exl = null;
private Excel.Workbook Workbook = null;
private static string DateTimeTemplate = "ДД.ММ.ГГГГ чч:мм:сс";
private object missing = Missing.Value;
private object read_only = true;
...
//реализация функций класса
this.exl = new Excel.ApplicationClass();
//this.PathToFile - путь к файлу какой-то
this.Workbook = this.exl.Workbooks.Open(
this.PathToFile
,this.missing
,this.read_only
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing
,this.missing);
//this.FirstDateCell
//, this.FirstFactorCell
//, this.LastDateCell
//, this.LastFactorCell - какие-то номера - координаты ячеек
this.exl.get_Range(
this.exl.Cells[this.FirstDateCell,this.FirstFactorCell]
,this.exl.Cells[this.LastDateCell,this.LastFactorCell]
).Select();
((Excel.Range)this.exl.Selection).NumberFormat = 
ImportFromExcel.DateTimeTemplate;