Update uptime stat using the input method.

def update(self):
        """Update uptime stat using the input method."""
        # Init new stats
        stats = self.get_init_value()

        if self.input_method == 'local':
            # Update stats using the standard system lib
            self.uptime = datetime.now() - datetime.fromtimestamp(psutil.boot_time())

            # Convert uptime to string (because datetime is not JSONifi)
            stats = str(self.uptime).split('.')[0]
        elif self.input_method == 'snmp':
            # Update stats using SNMP
            uptime = self.get_stats_snmp(snmp_oid=snmp_oid)['_uptime']
            try:
                # In hundredths of seconds
                stats = str(timedelta(seconds=int(uptime) / 100))
            except Exception:
                pass

        # Update the stats
        self.stats = stats

        return self.stats