'Schedule the job every day at a specific time.
Calling this is only valid for jobs scheduled to run
every N day(s).
:param time_str: A string in `XX:YY` format.
:return: The invoked job instance'

def at(self, time_str):

    assert ((self.unit in ('days', 'hours')) or self.start_day)
    (hour, minute) = time_str.split(':')
    minute = int(minute)
    if ((self.unit == 'days') or self.start_day):
        hour = int(hour)
        assert (0 <= hour <= 23)
    elif (self.unit == 'hours'):
        hour = 0
    assert (0 <= minute <= 59)
    self.at_time = datetime.time(hour, minute)
    return self
