Compute the instant when this job should run next.

def _schedule_next_run(self):
        """
        Compute the instant when this job should run next.
        """
        if self.unit not in ('seconds', 'minutes', 'hours', 'days', 'weeks'):
            raise ScheduleValueError('Invalid unit')

        if self.latest is not None:
            if not (self.latest >= self.interval):
                raise ScheduleError('`latest` is greater than `interval`')
            interval = random.randint(self.interval, self.latest)
        else:
            interval = self.interval

        self.period = datetime.timedelta(**{self.unit: interval})
        self.next_run = datetime.datetime.now() + self.period
        if self.start_day is not None:
            if self.unit != 'weeks':
                raise ScheduleValueError('`unit` should be \'weeks\'')
            weekdays = (
                'monday',
                'tuesday',
                'wednesday',
                'thursday',
                'friday',
                'saturday',
                'sunday'
            )
            if self.start_day not in weekdays:
                raise ScheduleValueError('Invalid start day')
            weekday = weekdays.index(self.start_day)
            days_ahead = weekday - self.next_run.weekday()
            if days_ahead <= 0:  # Target day already happened this week
                days_ahead += 7
            self.next_run += datetime.timedelta(days_ahead) - self.period
        if self.at_time is not None:
            if (self.unit not in ('days', 'hours', 'minutes')
                    and self.start_day is None):
                raise ScheduleValueError(('Invalid unit without'
                                          ' specifying start day'))
            kwargs = {
                'second': self.at_time.second,
                'microsecond': 0
            }
            if self.unit == 'days' or self.start_day is not None:
                kwargs['hour'] = self.at_time.hour
            if self.unit in ['days', 'hours'] or self.start_day is not None:
                kwargs['minute'] = self.at_time.minute
            self.next_run = self.next_run.replace(**kwargs)
            # If we are running for the first time, make sure we run
            # at the specified time *today* (or *this hour*) as well
            if not self.last_run:
                now = datetime.datetime.now()
                if (self.unit == 'days' and self.at_time > now.time() and
                        self.interval == 1):
                    self.next_run = self.next_run - datetime.timedelta(days=1)
                elif self.unit == 'hours' \
                        and self.at_time.minute > now.minute \
                        or (self.at_time.minute == now.minute
                            and self.at_time.second > now.second):
                    self.next_run = self.next_run - datetime.timedelta(hours=1)
                elif self.unit == 'minutes' \
                        and self.at_time.second > now.second:
                    self.next_run = self.next_run - \
                                    datetime.timedelta(minutes=1)
        if self.start_day is not None and self.at_time is not None:
            # Let's see if we will still make that time we specified today
            if (self.next_run - datetime.datetime.now()).days >= 7:
                self.next_run -= self.period