def at(self, time_str):
        """
        Specify a particular time that the job should be run at.

        :param time_str: A string in one of the following formats: `HH:MM:SS`,
            `HH:MM`,`:MM`, `:SS`. The format must make sense given how often
            the job is repeating; for example, a job that repeats every minute
            should not be given a string in the form `HH:MM:SS`. The difference
            between `:MM` and `:SS` is inferred from the selected time-unit
            (e.g. `every().hour.at(':30')` vs. `every().minute.at(':30')`).
        :return: The invoked job instance
        """
        if (self.unit not in ('days', 'hours', 'minutes')
                and not self.start_day):
            raise ScheduleValueError('Invalid unit')
        if not isinstance(time_str, str):
            raise TypeError('at() should be passed a string')
        if self.unit == 'days' or self.start_day:
            if not re.match(r'^([0-2]\d:)?[0-5]\d:[0-5]\d$', time_str):
                raise ScheduleValueError('Invalid time format')
        if self.unit == 'hours':
            if not re.match(r'^([0-5]\d)?:[0-5]\d$', time_str):
                raise ScheduleValueError(('Invalid time format for'
                                          ' an hourly job'))
        if self.unit == 'minutes':
            if not re.match(r'^:[0-5]\d$', time_str):
                raise ScheduleValueError(('Invalid time format for'
                                          ' a minutely job'))
        time_values = time_str.split(':')
        if len(time_values) == 3:
            hour, minute, second = time_values
        elif len(time_values) == 2 and self.unit == 'minutes':
            hour = 0
            minute = 0
            _, second = time_values
        else:
            hour, minute = time_values
            second = 0
        if self.unit == 'days' or self.start_day:
            hour = int(hour)
            if not (0 <= hour <= 23):
                raise ScheduleValueError('Invalid number of hours')
        elif self.unit == 'hours':
            hour = 0
        elif self.unit == 'minutes':
            hour = 0
            minute = 0
        minute = int(minute)
        second = int(second)
        self.at_time = datetime.time(hour, minute, second)
        return self