node-red mqtt roundtrip updated

mqtt-roundtrip.json

[
    {
        "id": "c67af34f.cfd1",
        "type": "function",
        "z": "9b7d2645.a72f18",
        "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    console.log(n-prev);\n}\n\nreturn msg;",
        "outputs": 1,
        "noerr": 0,
        "x": 382.5,
        "y": 241,
        "wires": [
            [
                "da5c6847.6b87f8"
            ]
        ]
    }
]