Date format representation for "2020-01-29T21:40:00.000+0000" to assign to @timestamp

[xyz2]

HI,

I need to assign the string in format "2020-01-29T21:40:00.000+0000" to @timestamp.
Used date filter . How is the format represented exactly ?
date {

  match => [ "[sampleDate]",  "ISO8601"]
  #match => [ "[sampleDate]",  "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'" ]

}

[Badger]

If [sampleDate] contains "2020-01-29T21:40:00.000+0000" then that date filter will produce

"@timestamp" => 2020-01-29T21:40:00.000Z,

[xyz2]

In the next step i try to do the following to assign valid timestamp to sampleDate

date {
match => [ "[sampleDate]", "ISO8601"]
target => "sampleDate"
}

and have the type for sampleDate as "date" in mapping template. However it errors out.
ERROR below:

"mapper_parsing_exception", "reason"=>"failed to parse field [sampleDate] of type [long] in document with id '9460474694a76b4d1823bf06805f9d47cc2701509279045fb07acbcab31c154eaead59d98825a39ae409aa2cc75b7a1f52802a8c394afc83c68749354a0539d1'. Preview of field's value: '2020-01-30T19:31:40.000Z'", "caused_by"=>{"type"=>"illegal_argument_exception", "reason"=>"For input string: "2020-01-30T19:31:40.000Z""}}}}}

NOTE: I also tried mutate copy @timestamp(after mapping) to sampleDate but that errors out too while indexing the data to ES