Ruby / Rails - 更改时间的时区，而不更改该值 [英] Ruby / Rails - Change the timezone of a Time, without changing the value

问题描述

我有一个记录 foo 在数据库中有：start_time 和：timezone 属性

I have a record foo in the database which has :start_time and :timezone attributes.

：start_time 是UTC的时间 - 2001-01-01 14:20:00 。
例如：timezone 是一个字符串 - America / New_York 。

The :start_time is a Time in UTC - 2001-01-01 14:20:00, for example. The :timezone is a string - America/New_York, for example.

我想创建一个新的Time对象，其值为：start_time ，但其时区由指定：timezone 。我不想加载：start_time 然后转换为：timezone ，因为Rails将会很聪明并且更新来自UTC的时间与该时区一致。

I want to create a new Time object with the value of :start_time but whose timezone is specified by :timezone. I do not want to load the :start_time and then convert to :timezone, because Rails will be clever and update the time from UTC to be consistent with that timezone.

目前，

t = foo.start_time
=> 2000-01-01 14:20:00 UTC
t.zone
=> "UTC"
t.in_time_zone("America/New_York")
=> Sat, 01 Jan 2000 09:20:00 EST -05:00

相反，我想看看

=> Sat, 01 Jan 2000 14:20:00 EST -05:00

ie。我想做：

t
=> 2000-01-01 14:20:00 UTC
t.zone = "America/New_York"
=> "America/New_York"
t
=> 2000-01-01 14:20:00 EST

推荐答案

听起来像你想要的东西沿着

Sounds like you want something along the lines of

ActiveSupport::TimeZone.new('America/New_York').local_to_utc(t)

这表示将本地时间（使用区域）转换为utc。如果你有 Time.zone ，那么你当然可以

This says convert this local time (using the zone) to utc. If you have Time.zone set then you can of course to

Time.zone.local_to_utc(t)

这不会使用附加到t的时区 - 它假设这是您正在转换的时区的本地。

This won't use the timezone attached to t - it assumes that it's local to the time zone you are converting from.

在这里保护的一个边缘案例是DST转换：您指定的本地时间可能不存在或可能不明确。

One edge case to guard against here is DST transitions: the local time you specify may not exist or may be ambiguous.

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