如何从给定的瞬间获得两个午夜？ [英] How can I get two midnights from given instant?

问题描述

我需要计算今天的00:00 AM和明天的00:00 AM。

I need to calculate 00:00 AM of today and 00:00 AM of tomorrow.

我想尝试这样

private static void some(final Date now) {
    final Calendar calendar = Calendar.getInstance();
    calendar.setTime(now);
    calendar.set(Calendar.MILLISECOND, 0);
    calendar.set(Calendar.SECOND, 0);
    calendar.set(Calendar.MINUTE, 0);
    calendar.set(Calendar.HOUR_OF_DAY, 0);
    final Date min = calendar.getTime(); // 00:00 AM of today
    calendar.add(Calendar.DATE, 1);
    final Date max = calendar.getTime(); // 00:00 AM of tomorrow
}

是否有更好的）

推荐答案

有什么问题需要解决。

首先，today对于 Date 不是一个明确定义的概念。 Date 基本上只是一个包装器自从Unix纪元以来的毫秒数：对应于该时刻的日历日期是不同的，取决于你在哪个时区。

Firstly, "today" is not a well-defined concept for a Date. Date is basically just a wrapper around number of milliseconds since Unix epoch: the calendar date corresponding to that instant is different, depending upon which timezone you are in.

例如，新日期（1469584693000）代表的时间在2016-07-27在伦敦;但它是在纽约市的2016-07-26。

For instance, the instant represented by new Date(1469584693000) was on 2016-07-27 in London; but it was on 2016-07-26 in NYC.

当然，你可以依赖于JVM的默认时区，但这使得代码的行为依赖JVM的配置。

You can, of course, rely upon the JVM's default timezone, but this makes the behaviour of the code dependent upon the JVM's configuration.

其次，午夜不是总是存在的：eg在亚洲/加沙时区，夏令时从午夜开始，意味着时钟从一天的23:59:59跳到下一个的01:00:00请参阅 Ideone演示 ）。这就是为什么Java 8时间API具有 atStartOfDay ，而不是 atMidnight 。

Secondly, "midnight" isn't something that always exists: e.g. in the Asia/Gaza timezone, daylight savings starts at midnight, meaning that the clock jumps from 23:59:59 on one day to 01:00:00 on the next (see Ideone demo). This is why the Java 8 time API has methods called atStartOfDay, not atMidnight.

所以，你可以把这在一起，在Java 8，到：

So, you can put this together, in Java 8, to:

private static void some(final Date now, ZoneId zone) {
  Instant instant = now.toInstant();  // Convert from old legacy class to modern java.time class.
  ZonedDateTime zdt = instant.atZone(zone);  // Apply a time zone to the UTC value.
  LocalDate today = zdt.toLocalDate();  // Extract a date-only value, without time-of-day and without time zone.

  ZonedDateTime startOfDayToday = today.atStartOfDay(zone);  // Determine first moment of the day.
  ZonedDateTime startOfDayTomorrow = today.plusDays(1).atStartOfDay(zone);

  // ...
}

您可以直接将即时或 ZonedDateTime 传递给方法;您可以使用显式常量 ZoneId ，例如 ZoneId.of（UTC）。

Of course, you can directly pass in the Instant or ZonedDateTime to the method; and you can use an explicit constant ZoneId, e.g. ZoneId.of("UTC").

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