show autopilot time

def cmd_time(self, args):
        '''show autopilot time'''
        tusec = self.master.field('SYSTEM_TIME', 'time_unix_usec', 0)
        if tusec == 0:
            print("No SYSTEM_TIME time available")
            return
        print("%s (%s)\n" % (time.ctime(tusec * 1.0e-6), time.ctime()))