couchdb query/view on timestamp subfields

I have a couchdb document.. roughly like this (minimal)

{
  "_id": "225136308d6f95611e457c0f02f1b47a769c35e8",
  "_rev": "19-f2affa088d817ec924c6ca03c34ef1bf",
  "communications": [
    {
      "sequenceId": 1,
      "timestamp": "2019-09-18T05:43:10.412Z"
    },
    {
      "sequenceId": 2,
      "timestamp": "2019-09-18T05:48:37.407Z",
    }
}

I want to look for all documents that has a latest timestamp older than X, how do I create a view to do that seeing as the timestamps are not stored as year,month,day?

I am not sure about your requirement but you could try something like this

{
  "_id": "_design/sortby",
  "views": {
    "timestamp": {
      "map": "function (doc) {\n  if (doc && doc.communications && Array.isArray(doc.communications)) {\n    doc.communications.forEach(function (communication) {\n      if (communication && communication.timestamp && communication.sequenceId) {\n        var date = new Date(communication.timestamp);\n        emit([date.getFullYear(), date.getMonth(), date.getDate()], communication.sequenceId);\n      }\n    });\n  }\n}"
    }
  },
  "language": "javascript",
  "_rev": "5-12eed6d29b7947eda8caa68be0b17a23"
}

it will create out put view like

{
    "total_rows": 2,
    "offset": 0,
    "rows": [
        {
            "key": [
                2019,
                8,
                18
            ],
            "id": "225136308d6f95611e457c0f02f1b47a769c35e8",
            "value": 1
        },
        {
            "key": [
                2019,
                8,
                18
            ],
            "id": "225136308d6f95611e457c0f02f1b47a769c35e8",
            "value": 2
        }
    ]
}

You can search based on year , month , date