'start and end must be strings in the format accepted by date'

def __init__(self, start=None, end=None):

    if (start is not None):
        self.start = date_from_str(start)
    else:
        self.start = datetime.datetime.min.date()
    if (end is not None):
        self.end = date_from_str(end)
    else:
        self.end = datetime.datetime.max.date()
    if (self.start > self.end):
        raise ValueError((u'Date    range:    "%s"    ,    the    start    date    must    be    before    the    end    date' % self))
