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# \$1dateFromString
<a name="dateFromString"></a>

Amazon DocumentDB 中的`$dateFromString`聚合运算符允许您将日期时间字符串转换为日期对象。当您的数据存储为字符串但需要对数据执行基于日期的操作时，这很有用。

**参数**
+ `dateString`：表示日期和时间的字符串。
+ `format`:（可选）指定格式的字符串`dateString`。如果未提供，Amazon DocumentDB 将尝试以 ISO-8601 格式解析字符串。
+ `timezone`:（可选）指定时区的字符串。如果未提供，Amazon DocumentDB 将使用服务器的时区。
+ `onError`:（可选）指定转换失败时要采取的操作。可能的值是`'error'`（默认值，会引发错误）、`'null'`（返回`null`）或`'replace'`（用`onErrorMessage`选项中指定的替换字符串替换值）。
+ `onErrorMessage`:（可选）如果设置`onError`为`'replace'`，则此选项指定替换字符串。

## 示例（MongoDB 外壳）
<a name="dateFromString-examples"></a>

以下示例演示了`$dateFromString`如何使用在 Amazon DocumentDB 中将日期字符串转换为日期对象。

**创建示例文档**

```
db.missionLog.insertMany([
{ _id: 1, event: "missionStart", logDate: "2020-03-15T13:41:33"},
{ _id: 2, event: "jumpPoint1", logDate: "2020-03-15T13:45:34"},
{ _id: 3, event: "jumpPoint2", logDate: "2020-03-15T13:48:21"},
{ _id: 4, event: "jumpPoint3", logDate: "2020-03-15T13:52:09"},
{ _id: 5, event: "missionEnd", logDate: "2020-03-15T13:58:44"}
]);
```

**查询示例**

```
db.missionLog.aggregate([
  {
    $project: {
      event: '$event',
      logDate: {
        $dateFromString: {
          dateString: '$logDate'
        }
      }
    }
  }
]);
```

**输出**

```
[
  {
    "_id": 1,
    "event": "missionStart",
    "logDate": ISODate("2020-03-15T13:41:33Z")
  },
  {
    "_id": 2,
    "event": "jumpPoint1",
    "logDate": ISODate("2020-03-15T13:45:34Z")
  },
  {
    "_id": 3,
    "event": "jumpPoint2",
    "logDate": ISODate("2020-03-15T13:48:21Z")
  },
  {
    "_id": 4,
    "event": "jumpPoint3",
    "logDate": ISODate("2020-03-15T13:52:09Z")
  },
  {
    "_id": 5,
    "event": "missionEnd",
    "logDate": ISODate("2020-03-15T13:58:44Z")
  }
]
```

## 代码示例
<a name="dateFromString-code"></a>

要查看使用该`$dateFromString`命令的代码示例，请选择要使用的语言的选项卡：

------
#### [ Node.js ]

```
const { MongoClient } = require('mongodb');

async function example() {
  const client = await MongoClient.connect('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false');
  const db = client.db('test');
  const collection = db.collection('missionLog');

  const result = await collection.aggregate([
    {
      $project: {
        event: '$event',
        logDate: {
          $dateFromString: {
            dateString: '$logDate'
          }
        }
      }
    }
  ]).toArray();

  console.log(result);
  await client.close();
}

example();
```

------
#### [ Python ]

```
from pymongo import MongoClient

def example():
    client = MongoClient('mongodb://<username>:<password>@<cluster-endpoint>:27017/?tls=true&tlsCAFile=global-bundle.pem&replicaSet=rs0&readPreference=secondaryPreferred&retryWrites=false')
    db = client['test']
    collection = db['missionLog']

    result = list(collection.aggregate([
        {
            '$project': {
                'event': '$event',
                'logDate': {
                    '$dateFromString': {
                        'dateString': '$logDate'
                    }
                }
            }
        }
    ]))

    print(result)
    client.close()

example()
```

------