Unable to convert timestamp to Date field

@magnusbaeck Hi, Hope this helps

      "tags" => [
    [0] "multiline",
    [1] "_dateparsefailure"
],
      "path" => "C:\\Users\\186\\Downloads\\xml_dev\\27.xml",
"@timestamp" => 2018-02-20T18:04:05.641Z,
       "avg" => [
    [ 0] "5262.85302734375",
    [ 1] "4866.47314453125",
    [ 2] "5032.392822265625",
    [ 3] "4062.9139404296875",
    [ 4] "4947.111328125",
    [ 5] "4758.715087890625",
    [ 6] "3081.375244140625",
    [ 7] "4437.6788330078125",
    [ 8] "7593.035888671875",
    [ 9] "15168.4794921875",
    [10] "4901.086669921875",
    [11] "4973.13232421875",
    [12] "5004.08642578125",
    [13] "4736.31689453125",
    [14] "4944.011474609375",
    [15] "4908.229248046875",
    [16] "4778.435302734375",
    [17] "4489.117919921875"
],
       "min" => [
    [ 0] 5262,
    [ 1] 4866,
    [ 2] 5032,
    [ 3] 4062,
    [ 4] 4947,
    [ 5] 4758,
    [ 6] 3081,
    [ 7] 4437,
    [ 8] 7593,
    [ 9] 15168,
    [10] 4901,
    [11] 4973,
    [12] 5004,
    [13] 4736,
    [14] 4944,
    [15] 4908,
    [16] 4778,
    [17] 4489
],
  "@version" => "1",
      "host" => "B433",
   "bt_name" => [
    [0] "test"
],
 "timestamp" => [
    [ 0] 1518044400000,
    [ 1] 1518087600000,
    [ 2] 1518109200000,
    [ 3] 1518130800000,
    [ 4] 1518174000000,
    [ 5] 1518195600000,
    [ 6] 1518217200000,
    [ 7] 1518260400000,
    [ 8] 1518368400000,
    [ 9] 1518411600000,
    [10] 1518433200000,
    [11] 1518454800000,
    [12] 1518476400000,
    [13] 1518519600000,
    [14] 1518541200000,
    [15] 1518562800000,
    [16] 1518606000000,
    [17] 1518627600000
]

}
{
"@version" => "1",
"host" => "B433",
"path" => "C:\Users\186\Downloads\xml_dev\27.xml",
"@timestamp" => 2018-02-20T18:04:05.642Z,
"message" => " "
}
{
"@version" => "1",
"host" => "B433",
"path" => "C:\Users\186\Downloads\xml_dev\27.xml",
"@timestamp" => 2018-02-20T18:04:05.643Z,
"message" => " "
}
{
"@version" => "1",
"host" => "B433",
"path" => "C:\Users\186\Downloads\xml_dev\27.xml",
"@timestamp" => 2018-02-20T18:04:05.643Z,
"message" => " "
}
{
"@version" => "1",
"host" => "B433",
"path" => "C:\Users\186\Downloads\xml_dev\27.xml",
"@timestamp" => 2018-02-20T18:04:05.644Z,
"message" => " "
}