$dateFromString (aggregation operator)（聚合运算符）

Converting Dates转换日期

Consider a collection logmessages that contains the following documents with dates.考虑一个包含以下带日期的文档的集合logmessages。

{ _id: 1, date: "2017-02-08T12:10:40.787", timezone: "America/New_York", message:  "Step 1: Started" },
{ _id: 2, date: "2017-02-08", timezone: "-05:00", message:  "Step 1: Ended" },
{ _id: 3, message:  " Step 1: Ended " },
{ _id: 4, date: "2017-02-09", timezone: "Europe/London", message: "Step 2: Started"},
{ _id: 5, date: "2017-02-09T03:35:02.055", timezone: "+0530", message: "Step 2: In Progress"}

The following aggregation uses $dateFromString to convert the date value to a date object:以下聚合使用$dateFromString将date值转换为日期对象：

db.logmessages.aggregate( [ {
   $project: {
      date: {
         $dateFromString: {
            dateString: '$date',
            timezone: 'America/New_York'
         }
      }
   }
} ] )

The above aggregation returns the following documents and converts each date field to the Eastern Time Zone:上述聚合返回以下文档，并将每个date字段转换为东部时区：

{ "_id" : 1, "date" : ISODate("2017-02-08T17:10:40.787Z") }
{ "_id" : 2, "date" : ISODate("2017-02-08T05:00:00Z") }
{ "_id" : 3, "date" : null }
{ "_id" : 4, "date" : ISODate("2017-02-09T05:00:00Z") }
{ "_id" : 5, "date" : ISODate("2017-02-09T08:35:02.055Z") }

The timezone argument can also be provided through a document field instead of a hard coded argument. For example:timezone参数也可以通过文档字段而不是硬编码参数提供。例如：

db.logmessages.aggregate( [ {
   $project: {
      date: {
         $dateFromString: {
            dateString: '$date',
            timezone: '$timezone'
         }
      }
   }
} ] )

The above aggregation returns the following documents and converts each date field to their respective UTC representations.上述聚合返回以下文档，并将每个date字段转换为各自的UTC表示。

{ "_id" : 1, "date" : ISODate("2017-02-08T17:10:40.787Z") }
{ "_id" : 2, "date" : ISODate("2017-02-08T05:00:00Z") }
{ "_id" : 3, "date" : null }
{ "_id" : 4, "date" : ISODate("2017-02-09T00:00:00Z") }
{ "_id" : 5, "date" : ISODate("2017-02-08T22:05:02.055Z") }

onError

If your collection contains documents with unparsable date strings, $dateFromString throws an error unless you provide an aggregation expression to the optional onError parameter.如果集合包含具有不可解析日期字符串的文档，除非您为可选的onError参数提供聚合表达式，否则$dateFromString将抛出错误。

For example, given a collection dates with the following documents:例如，给定一个包含以下文档的集合dates：

{ "_id" : 1, "date" : "2017-02-08T12:10:40.787", timezone: "America/New_York" },
{ "_id" : 2, "date" : "20177-02-09T03:35:02.055", timezone: "America/New_York" }

You can use the onError parameter to return the invalid date in its original string form:您可以使用onError参数以原始字符串形式返回无效日期：

db.dates.aggregate( [ {
   $project: {
      date: {
         $dateFromString: {
            dateString: '$date',
            timezone: '$timezone',
            onError: '$date'
         }
      }
   }
} ] )

This returns the following documents:这将返回以下文档：

{ "_id" : 1, "date" : ISODate("2017-02-08T17:10:40.787Z") }
{ "_id" : 2, "date" : "20177-02-09T03:35:02.055" }

onNull

If your collection contains documents with null date strings, $dateFromString returns null unless you provide an aggregation expression to the optional onNull parameter.如果集合包含日期字符串为null的文档，除非您为可选的onNull参数提供聚合表达式，否则$dateFromString将返回null。

For example, given a collection dates with the following documents:例如，给定一个包含以下文档的集合dates：

{ "_id" : 1, "date" : "2017-02-08T12:10:40.787", timezone: "America/New_York" },
{ "_id" : 2, "date" : null, timezone: "America/New_York" }

You can use the onNull parameter to have $dateFromString return a date representing the unix epoch instead of null:您可以使用onNull参数让$dateFromString返回一个表示unix纪元的日期，而不是null：

db.dates.aggregate( [ {
   $project: {
      date: {
         $dateFromString: {
            dateString: '$date',
            timezone: '$timezone',
            onNull: new Date(0)
         }
      }
   }
} ] )

This returns the following documents:这将返回以下文档：

{ "_id" : 1, "date" : ISODate("2017-02-08T17:10:40.787Z") }
{ "_id" : 2, "date" : ISODate("1970-01-01T00:00:00Z") }