node-red MQTT roundtrip latency calculator

MQTTtest.json

[
    {
        "id": "e0241b25.e1c018",
        "type": "mqtt in",
        "z": "eeecba5a.679b68",
        "name": "",
        "topic": "/pishield/pingtest2",
        "qos": "1",
        "broker": "858b78cb.00a038",
        "x": 194,
        "y": 225,
        "wires": [
            [
                "f3b4dc48.7a2b2"
            ]
        ]
    },
    {
        "id": "fa833e4a.530df",
        "type": "mqtt out",
        "z": "eeecba5a.679b68",
        "name": "",
        "topic": "/pishield/pingtest2",
        "qos": "1",
        "retain": "",
        "broker": "858b78cb.00a038",
        "x": 502,
        "y": 97,
        "wires": []
    },
    {
        "id": "e98937ae.598f98",
        "type": "inject",
        "z": "eeecba5a.679b68",
        "name": "",
        "topic": "timestamp",
        "payload": "",
        "payloadType": "date",
        "repeat": ".1",
        "crontab": "",
        "once": false,
        "x": 258.5,
        "y": 97,
        "wires": [
            [
                "fa833e4a.530df",
                "f3b4dc48.7a2b2"
            ]
        ]
    },
    {
        "id": "faaa021b.feef4",
        "type": "debug",
        "z": "eeecba5a.679b68",
        "name": "",
        "active": false,
        "console": "false",
        "complete": "false",
        "x": 572.5,
        "y": 203,
        "wires": []
    },
    {
        "id": "f3b4dc48.7a2b2",
        "type": "function",
        "z": "eeecba5a.679b68",
        "name": "deltaT",
        "func": "\nvar prev = flow.get('prev')||0;\n\nvar d = new Date();\nvar n = d.getMilliseconds();\n\nif (msg.topic == \"timestamp\"){\n    flow.set('prev', n);\n}\n   \nelse {\n    if (n<prev) {\n        console.log(\"OVERFLOW!\");\n        n=n+1000;\n    }\n    console.log(\"tDiff = \",n,\"-\",prev,\"=\",n-prev);\n}\n\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "x": 386.5,
        "y": 226,
        "wires": [
            [
                "faaa021b.feef4"
            ]
        ]
    },
    {
        "id": "858b78cb.00a038",
        "type": "mqtt-broker",
        "z": "",
        "broker": "raspberrypi.local",
        "port": "1883",
        "clientid": "",
        "usetls": false,
        "compatmode": true,
        "keepalive": "60",
        "cleansession": true,
        "willTopic": "",
        "willQos": "0",
        "willPayload": "",
        "birthTopic": "",
        "birthQos": "0",
        "birthPayload": ""
    }
]