如何使用elasticsearch php 查詢當前index中，時間字段的最大值？

我有一個gitlab_commit_index, 包含了30萬條document，其中每個document都有一個commit_date字段，我想使用elasticsearch php 查詢這個index中commit_date的最大值是多少，請教各位怎樣查，下面是我的錯誤寫法：
$params = [
'index' => 'gitlab_commit_index',
'type' => 'gitlab_commit_type',
'body' => [
'aggs' => [
'max_commit_date' => [
'max' => [
[
"field" => "commitDate"
],
],
],
]
],

];
報錯信息：
{"error":{"root_cause":[{"type":"parsing_exception","reason":"Expected [START_OBJECT] under [max], but got a [START_ARRAY] in [max_commit_date]","line":1,"col":35}],"type":"parsing_exception","reason":"Expected [START_OBJECT] under [max], but got a [START_ARRAY] in [max_commit_date]","line":1,"col":35},"status":400}