使用NSDateFormatter将UTC转换为本地时间 [英] Convert UTC to local time with NSDateFormatter

问题描述

我正在从iOS应用中的服务器上获取以下字符串：

I am getting the following string from a server in my iOS app:

20140621-061250

20140621-061250

如何将其转换为当地时间？

How can I convert it to the local time?

如何定义日期格式？这是正确的吗？

How can I define my date formatter? Is this correct?

dateFormatter.dateFormat = @"YYYYMMd-HHmmss"; 

推荐答案

这个问题并没有说明你所要表达的本质意味着要进行精确转换，但是无论最终目标如何，您要做的第一件事是使用正确配置的 NSDateFormatter 正确解析服务器响应。这要求指定正确的格式字符串，并且必须在格式器上显式设置时区，否则它将从本地时间推断出来，这在大多数情况下是不正确的。

The question doesn't specify the nature of what you mean by converting, exactly, but the first thing you should do, regardless of the final goal, is to correctly parse the server response using a properly configured NSDateFormatter. This requires specification of the correct format string, and the time zone must be explicitly set on the formatter or it will infer it from the local time, which would be incorrect in most cases.

让我们看一下提供的输入字符串：

Let's look at the input string provided:

20140621-061250

20140621-061250

此年份使用四位数字，月份使用两位数字（填充为零），并且当天的两位数（大概也将被补零）。然后是-，然后用两位数字表示小时，用2位数字表示分钟，用2位数字表示秒。

This uses four digits for the year, two digits (with a zero-padding) for the month, and two digits (presumably, these will be zero-padded as well) for the day. This is followed by a -, then two digits to represent the hour, 2 digits for the minute, and 2 digits for the second.

引用 Unicode日期格式标准，我们可以通过以下方式导出格式字符串。代表日历年的四位数将被格式字符串中的 yyyy 代替。月份使用 MM ，一天使用 dd 。接下来是文字-。对于小时，我假设它将采用24小时格式，否则此响应会模棱两可，因此我们使用 HH 。分钟为 mm ，秒为。连接格式说明符会产生以下格式字符串，我们将在下一步中使用：

Referring to the Unicode date format standards, we can derive the format string in the following way. The four digits representing the calendar year will be replaced with yyyy in the format string. Use MM for the month, and dd for the day. Next would come the literal -. For the hours, I assume that it will be in 24 hour format as otherwise this response is ambiguous, so we use HH. Minutes are then mm and seconds ss. Concatenating the format specifiers yields the following format string, which we will use in the next step:

yyyyMMdd-HHmmss

在我们的程序中，它看起来像是：

In our program, this would look like:

NSString *dateFormat = @"yyyyMMdd-HHmmss";

配置输入日期格式程序

上面的时间格式没有指定时区，但是因为已经为您提供了代表UTC时间的服务器响应的规范，所以我们可以将其编码到应用程序中。因此，我们实例化一个 NSDateFormatter ，设置正确的时区并设置日期格式：

Configure the input date formatter

The time format above does not specify a time zone, but because you have been provided the specification for the server response that it represents the UTC time, we can code this into our application. So, we instantiate an NSDateFormatter, set the correct time zone, and set the date format:

NSTimeZone *inputTimeZone = [NSTimeZone timeZoneWithAbbreviation:@"UTC"];
NSDateFormatter *inputDateFormatter = [[NSDateFormatter alloc] init];
[inputDateFormatter setTimeZone:inputTimeZone];
[inputDateFormatter setDateFormat:dateFormat];

将输入字符串转换为 NSDate

出于演示目的，我们对您从服务器响应中收到的字符串进行了硬编码；您将用从服务器获得的定义替换 inputString 的定义：

NSString *inputString = @"20140621-061250";
NSDate *date = [inputDateFormatter dateFromString:inputString];

此时，我们有必要的对象来进行任何进一步的转换或计算- NSDate 代表服务器传达的时间。请记住， NSDate 只是一个时间戳-它与时区没有任何关系，它仅在与日期的字符串表示形式相互转换时才起作用，或者通过 NSDateComponents 表示日历日期。

At this point, we have the necessary object to do any further conversions or calculations - an NSDate which represents the time communicated by the server. Remember, an NSDate is just a time stamp - it has no relation to a time zone whatsoever, which only plays a role when converting to and from string representations of the date, or representations of a calendrical date via NSDateComponents.

这个问题并没有明确说明需要哪种类型的转换，因此我们将看到一个格式化日期的示例，使其以与服务器响应相同的格式显示（尽管我无法想到可能的用法）老实说，这种特殊代码的大小写）。步骤非常相似-我们指定格式字符串，时区，配置日期格式器，然后从日期生成字符串（以指定格式）：

The question doesn't clearly specify what type of conversion is needed, so we'll see an example of formatting the date to display in the same format as the server response (although, I can't think of a likely use case for this particular bit of code, to be honest). The steps are quite similar - we specify a format string, a time zone, configure a date formatter, and then generate a string (in the specified format) from the date:

NSTimeZone *outputTimeZone = [NSTimeZone localTimeZone];
NSDateFormatter *outputDateFormatter = [[NSDateFormatter alloc] init];
[outputDateFormatter setTimeZone:outputTimeZone];
[outputDateFormatter setDateFormat:dateFormat];
NSString *outputString = [outputDateFormatter stringFromDate:date];

因为我在UTC-06：00，所以打印 outputString 给出以下内容：

Since I'm in UTC-06:00, printing outputString gives the following:

20140621-001250

20140621-001250

您可能会改用 setDateStyle：和 setTimeStyle：格式字符串（如果要向用户显示此日期），或使用 NSCalendar 获取 NSDateComponents 实例来执行日期上的算术或计算。向用户显示详细日期字符串的示例：

It's likely you'll instead want to use setDateStyle: and setTimeStyle: instead of a format string if you're displaying this date to the user, or use an NSCalendar to get an NSDateComponents instance to do arithmetic or calculations on the date. An example for displaying a verbose date string to the user:

NSTimeZone *outputTimeZone = [NSTimeZone localTimeZone];
NSDateFormatter *outputDateFormatter = [[NSDateFormatter alloc] init];
[outputDateFormatter setTimeZone:outputTimeZone];
[outputDateFormatter setDateStyle:NSDateFormatterFullStyle];
[outputDateFormatter setTimeStyle:NSDateFormatterFullStyle];
NSString *outputString = [outputDateFormatter stringFromDate:date];

打印 outputString 可以给我们以下内容：

Printing outputString here gives us the following:

2014年6月21日，星期六，山区夏令时

Saturday, June 21, 2014 at 12:12:50 AM Mountain Daylight Time

请注意，适当设置时区将处理夏令时期间的过渡。使用上面的格式化程序样式代码将输入字符串更改为 20141121-061250，将为我们显示以下日期（请注意，山峰标准时间为UTC-7）：

Note that setting the time zone appropriately will handle transitions over daylight savings time. Changing the input string to "20141121-061250" with the formatter style code above gives us the following date to display (Note that Mountain Standard Time is UTC-7):

山区标准时间2014年11月20日星期四11:12:50

Thursday, November 20, 2014 at 11:12:50 PM Mountain Standard Time

每当您以代表日历日期和时间的字符串形式获取日期输入时，第一步就是使用 NSDateFormatter 根据输入源和您的要求，为输入的格式，时区以及可能的语言环境和日历进行配置。这将产生一个 NSDate ，它是时间的明确表示。创建完 NSDate 之后，可以根据应用程序的需要对其进行格式化，设置样式或将其转换为日期组件。

Summary

Any time you get date input in a string form representing a calendar date and time, your first step is to convert it using an NSDateFormatter configured for the input's format, time zone, and possibly locale and calendar, depending on the source of the input and your requirements. This will yield an NSDate which is an unambiguous representation of a moment in time. Following the creation of that NSDate, one can format it, style it, or convert it to date components as needed for your application requirements.

这篇关于使用NSDateFormatter将UTC转换为本地时间的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！