为什么是Date.today - 6.months + 6.months！= Date.today？ [英] Why is Date.today - 6.months + 6.months != Date.today?

问题描述

在Ruby中，在万圣节：

In Ruby, on Halloween:

Date.today - 6.months + 6.months != Date.today

我们需要更新Ruby的日期实现吗？其他语言有同样的问题吗？

Do we need to update Ruby's date implementation? Do other languages have the same issue?

推荐答案

如果您进行任何没有31天的任何月份，即3个月就可以正常工作，但是1个月，或6个，或8个都会发生这种情况）。

This happens if you do it to any month that doesn't have 31 days (i.e. 3 months would work just fine, but 1 month, or 6, or 8 would all make this happen).

如果你执行 Date。今天 - 1.month ，看起来Rails看到9/31/2011是不是有效的日期，所以它踢了一个额外的一天，使其成为有效的日期。但是，当您从9月底开始一个月前，将于10/30/2011离开，因为这是有效的日期。基本上，Rails只是尝试增加（或减少）月份字段，只要它是有效的日期，它将不会调整日期字段。

If you do Date.today - 1.month, it looks like Rails sees that 9/31/2011 isn't a valid date, so it kicks it back an extra day to make it a valid date. However, when you go one month forward from the end of September, it'll leave it at 10/30/2011 since that's a valid date. Basically, Rails just tries to increment (or decrement) the month field and as long as it's a valid date, it won't adjust the day field.

一种方法解决这个问题的方法是在日期上使用 .end_of_month （或 .beginning_of_month ）方法对象在Rails中，以确保您一直得到结束或一个月的开始。

One way to work around this is to use the .end_of_month (or .beginning_of_month) method on a Date object in Rails to make sure you're consistently getting the end or beginning of a month.

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