'Transform unix :attr:`timestamp` to a data string in ISO format.
.. note:: This is a `classmethod`.
:param timestamp: A unix timestamp indicating a specified time.
:type timestamp: number
.. note:: The :attr:`timestamp` could be `int` or `float`.
:return: Date in ISO format, which is `YY-mm-dd` in specific.
:rtype: str'

@classmethod
def timestamp_to_date(cls, timestamp):

    l_time = time.localtime(float(timestamp))
    iso_format = '%Y-%m-%d'
    date = time.strftime(iso_format, l_time)
    return date
