问题描述
我有遗留代码，使用java.util.Date创建一个古老的日期（12月30日0002）。我正在尝试更新我可以使用的代码，但这需要在Date和LocalDate等之间进行转换。我无法完全摆脱使用Date或古老的日期选择。
I have legacy code that uses java.util.Date creating an ancient date (30 Nov 0002). I'm trying to update what code I can, but that's necessitating converting between Date and LocalDate, etc. I can't completely get rid of the use of Date or the ancient date choice.
我发现在日期和瞬间之间在这个古老的日期之间来回转换时出现了什么错误，并且希望有人可以解释发生了什么。
I'm finding what appears to be an error when converting back and forth between Date and Instant with this ancient date, and was hoping someone could explain what is going on.
以下是一个示例：
Date date = new Date();
Instant instant = date.toInstant();
System.out.println("Current:");
System.out.println("Date: "+date);
System.out.println("Instant: "+instant);
System.out.println("Date epoch: "+date.getTime());
System.out.println("Instant epoch: "+instant.getEpochSecond()*1000);
System.out.println("\nAncient from Date:");
Calendar cal = new GregorianCalendar(TimeZone.getTimeZone("MST"));
cal.set(2, Calendar.NOVEMBER, 30, 0, 0, 0);
date = cal.getTime();
instant = date.toInstant();
System.out.println("Date: "+date);
System.out.println("Instant: "+instant);
System.out.println("Date epoch: "+date.getTime());
System.out.println("Instant epoch: "+instant.getEpochSecond()*1000);
System.out.println("\nAncient from Instant:");
instant = Instant.parse("0002-11-30T00:00:00Z");
date = Date.from(instant);
System.out.println("Date: "+date);
System.out.println("Instant: "+instant);
System.out.println("Date epoch: "+date.getTime());
System.out.println("Instant epoch: "+instant.getEpochSecond()*1000);
打印以下内容：
Current:
Date: Tue Sep 18 12:34:27 MST 2018
Instant: 2018-09-18T19:34:27.177Z
Date epoch: 1537299267177
Instant epoch: 1537299267000
Ancient from Date:
Date: Thu Nov 30 00:00:00 MST 2
Instant: 0002-11-28T07:00:00.247Z
Date epoch: -62075437199753
Instant epoch: -62075437200000
Ancient from Instant:
Date: Fri Dec 01 17:00:00 MST 2
Instant: 0002-11-30T00:00:00Z
Date epoch: -62075289600000
Instant epoch: -62075289600000
因此，如果我在11月30日创建一个Instant，然后转换为Date，则日期是12月1日。如果我在11月30日开始日期，则Instant是11月28日2.我知道日期和即时存储时区信息都没有，但为什么这个时代根据我是从一个日期还是一个瞬间开始而如此不同？无论如何我可以解决这个问题吗？我需要能够以Date或Instant开头，并以相同的纪元值结束。也很高兴知道为什么默认的toString（）在给定相同的纪元时显示这样的不同日期。
So if I create an Instant at 30 Nov 2, then convert to a Date, the Date is 1 Dec 2. If I start with a Date at 30 Nov 2, the Instant is 28 Nov 2. I'm aware that neither Date nor Instant store timezone information, but why are the epochs so different based on whether I started with a Date vs. an Instant? Is there anyway I can work around that? I need to be able to start with either a Date or an Instant, and end up with the same epoch value. It would also be nice to know why the default toString() shows such different dates given the same epoch.
推荐答案
差异存在在 Date 和 Instant 的实现如何相互作用与其实现相互作用，使用Date使用Gregorian / Julian日历和Instant使用ISO标准的日期，遵循Julian日历切换之前修改过的公历。
The discrepancy resides in how the implementations of Date and Instant interact with each other in relation to their implementations, with Date using Gregorian/Julian calendars and Instant using ISO standard for Date, which follow a modified Gregorian calendar prior to the Julian calendar switchover.
GregorianCalendar 有一个特别说明：
The GregorianCalendar implementation has a special note:
嗯，是的从技术上讲。但是对于这个问题，我们不会完全遇到这个。
Well, yes, technically speaking. But for this issue, we don't quite encounter this.
cal.set(1582, Calendar.OCTOBER, 4, 0, 0, 0);
这将产生1582年10月4日的预期日期。
This yields a date, as expected, of October 4, 1582.
cal.set(1582, Calendar.OCTOBER, 5, 0, 0, 0);
这会产生10月 15 ，1582的日期。
This yields a date, of October 15, 1582.
这是什么魔法，蝙蝠侠？
WHAT MAGIC IS THIS, BATMAN?
嗯，这不是编码错误，它实际上是GregorianCalendar的一个实现。
Well, this isn't a coding error, it's actually an implementation of GregorianCalendar.
来自
当我们检查1582年10月4日时，会发生以下情况：
When we examine October 4, 1582, the following happens:
即时：1582-10-14T00：00：00Z
Instant: 1582-10-14T00:00:00Z
此处存在10天的差距，并且技术上不存在的日期中的瞬间存在的原因由。
There is a 10-day gap here, and the reason the instant exists on a "technically non-existent date" is accounted for by the definition of ISO instant dates.
SO，而1582年10月14日在现实中从未存在，按照定义存在于ISO时间，但发生在现实世界的10月根据朱利安历法，4，1582年。
SO, whereas October 14, 1582 never existed in reality, it exists in ISO time by definition, but occurs on the real world's October 4, 1582 according to Julian Calendar.
由于我假设是第一段的额外闰年漂移，其中朱利安世纪1500,1400,1300,1100,1000 ，900,700,600,500,300,200,100在格里高利历中没有计算额外的闰日，我们慢慢地从+10偏移到-1偏移。这可以通过以+100为增量调整年份来验证。
Due to what I assume are additional leap year drifts from the first paragraph, where Julian centuries 1500, 1400, 1300, 1100, 1000, 900, 700, 600, 500, 300, 200, 100 have extra leap days not accounted for in the Gregorian calendar, we slowly shift back from a +10 to a -1 offset. This can be verified by adjusting the year in +100 increments.
如果您显示历史事件日期，您最好使用日期或 JulianCalendar DateFormatter显示正确的历史日期，因为它实际上发生在历史记录中。打印历史时段的ISO时间可能看似荒谬或不准确，但以此格式存储时间仍然有效。
If you are displaying historical event dates, you will be better off using a Date or JulianCalendar DateFormatter to display the correct correct historical date, as it actually occurred in history. Printing out the ISO time for historical periods may appear nonsensical or inaccurate, but storing the time in this format is still valid.
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