def is_date(cells, valid_formats):
    """
    Does the Excel number format contain a date convertion?

    :param cells: A single cell with a value which could be a date
    :param valid_formats: Sequence of valid Excel date format strings
            e.g.  = ('dd/mm/yyyy',)
    :return: bool
    """
    try:
        float(cells.Value2)
    except ValueError:
        return False
    except TypeError:  # typically a tuple (range of cells)
        return False

    return cells.NumberFormat in valid_formats