def span_to_str(span):
    """ Return the time periodicity.

    Parameters
    ----------
    span : int
        Time interval in second.

    Returns
    -------
    date : str
        Time periodicity.

    Examples
    --------
    >>> span_to_str(3600)
    'Hourly'

    """
    if span == 60:
        return 'Minutely'
    elif span == 300:
        return 'Five_Minutely'
    elif span == 1800:
        return 'Half_Hourly'
    elif span == 3600:
        return 'Hourly'
    elif span == 7200:
        return 'Bi_Hourly'
    elif span == 86400:
        return 'Daily'
    elif span == 604800:
        return 'Weekly'
    else:
        print('Error, no string correspond to this time in seconds.')