Modify your definition of UltimaFetcha:

UltimaFetcha (new Date,"some text", ThisWorkbook.Sheets("sheetname")

Function UltimaFecha(Fecha As Date, celda As String, sheet as
WorkSheet) As Date
Application.Volatile
* Fecha = sheet.Cells(sheet.Cells.Rows.Count, celda).End(xlUp).Value
* UltimaFecha = Fecha
End Function