这些日期是什么类型的?

The second is the default formatting from the description method a NSDate (e.g. you NSLog a NSDate).

第三个是RFC 3339/ISO 8601 格式(只有几分之一秒)，通常用于 Web 服务.

The third is RFC 3339/ISO 8601 format (with fractions of a second), often used in web services.

请参阅 Apple 的 技术问答 QA1480.

See Apple's Technical Q&A QA1480.

顺便说一句，该技术说明不包括毫秒，因此您可能希望使用类似 @"yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'SSS'Z'"，例如:

As an aside, that Technical Note doesn't include the milliseconds, so you might want to use something like @"yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'SSS'Z'", for example:

NSDate *date = [NSDate date];
NSLocale *enUSPOSIXLocale = [[NSLocale alloc] initWithLocaleIdentifier:@"en_US_POSIX"];
NSDateFormatter *formatter = [[NSDateFormatter alloc] init];
formatter.locale = enUSPOSIXLocale;
formatter.dateFormat = @"yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'SSS'Z'";
formatter.timeZone = [NSTimeZone timeZoneForSecondsFromGMT:0];
NSString *dateString = [formatter stringFromDate:date];
NSLog(@"%@", dateString);

如果您想将日期作为 RFC 3339/ISO 8601 格式的字符串存储在 plist 中(或者，如果您需要将 NSDate 转换为字符串以进行传输，则可以使用它到您的网络服务).如上所述，默认的 plist 格式不会为 NSDate 对象保留几分之一秒，因此如果这很关键，将日期存储为上述代码生成的字符串可能会很有用.

You can use this if you want to store the date as a string in RFC 3339/ISO 8601 format in the plist (or alternatively, if you need to convert a NSDate to a string for transmission to your web service). As noted above, the default plist format does not preserve fractions of a second for NSDate objects, so if that's critical, storing dates as strings as generated by the above code can be useful.

这个日期格式化程序可用于将日期转换为字符串(使用 stringFromDate)，以及将正确格式化的字符串转换为 NSDate 对象(使用 dateFromString).

And this date formatter can be used for converting dates to strings (using stringFromDate), as well as converting properly formatting strings to NSDate objects (using dateFromString).

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