如何在过去的特定时间使用UTC创建Date对象？ [英] How to create a Date object, using UTC, at a specific time in the past?
问题描述
是否可以创建一个 java.util.Date 对象，该对象保证为UTC并且在过去的特定时间，例如一小时前或一天以前，这是一个好主意吗？
Is it possible to create a java.util.Date object that is guaranteed to be UTC and at a specific time in the past, like an hour ago or a day ago, and is this a good idea?
我查看了Stack Overflow问题
获取4小时前的日期及其答案。但我希望避免添加更多依赖项，例如 jodaTime ，并且接受的答案使用 System.currentTimeMillis（）这不是本地时区，不是吗？
I have reviewed the Stack Overflow question
get date as of 4 hours ago and its answers. But I want to avoid having to add more dependencies, like jodaTime, and the accepted answer uses System.currentTimeMillis() which would be the local timezone, would it not?
推荐答案
正如评论中生动地讨论的那样，建议使用 java.time 包。简单的解决方案：
As discussed vividly in the comments, the recommendation is to use the java.time package. The easy solution:
Instant fourHoursAgo = Instant.now().minus(Duration.ofHours(4));
System.out.println(fourHoursAgo);
这只是打印
2018-01-31T14:57:44.667255Z
由于现在是UTC时间 18:58 ，输出就是你要求的。 Instant 本身偏移中性。它的 toString 以UTC格式提供时间，但在生成 Instant 时没有提及UTC，所以它是否给你你想要什么，我不确定。我稍后会在UTC中给你一个显式的结果。
Since UTC time is now 18:58, the output is what you asked for. The Instant itself is offset neutral. Its toString delivers time in UTC, but there was no mention of UTC in producing the Instant, so whether it gives you what you want, I am not sure. I will give you a result that is explicitly in UTC later.
但首先，如果你需要一个 java .util.Date ，通常对于您无法更改的旧API，转换很简单：
But first, if you do need a java.util.Date, typically for a legacy API that you cannot change, the conversion is easy:
Date oldfashionedDate = Date.from(fourHoursAgo);
System.out.println(oldfashionedDate);
在我的电脑上欧洲/哥本哈根时间打印区域：
On my computer in Europe/Copenhagen time zone this printed:
Wed Jan 31 15:57:44 CET 2018
同样，这与运行代码段前四小时的时间一致。同样， Date 中没有UTC偏移量。只有它的 toString 方法才能获取我的JVM的时区设置并用它来生成字符串，这不会影响 Date 。请参阅Stack Overflow问题，如何设置时区一个java.util.Date？及其答案。
Again, this agrees with the time four hours before running the snippet. And again, a Date doesn’t have a UTC offset in it. Only its toString method grabs my JVM’s time zone setting and uses it for generating the string, this does not affect the Date. See the Stack Overflow question, How to set time zone of a java.util.Date?, and its answers.
正如所承诺的那样，如果你不仅需要代表时间而且还要代表偏移量，请使用 OffsetDateTime ：
As promised, if you do need to represent not only the time but also the offset, use an OffsetDateTime:
OffsetDateTime fourHoursAgoInUtc = OffsetDateTime.now(ZoneOffset.UTC).minusHours(4);
System.out.println(fourHoursAgoInUtc);
此印刷
2018-01-31T14:57:44.724147Z
Z 在结尾处意味着从UTC或祖鲁时区（这不是真正的时区）偏移零。转换为日期并不比以前复杂得多，但同样，您将失去转换中的偏移信息：
Z at the end means offset zero from UTC or "Zulu time zone" (which isn’t a true time zone). The conversion to a Date is not much more complicated than before, but again, you will lose the offset information in the conversion:
Date oldfashionedDate = Date.from(fourHoursAgoInUtc.toInstant());
System.out.println(oldfashionedDate);
此印刷：
Wed Jan 31 15:57:44 CET 2018
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