从UTC转换为本地时区会产生错误的结果 [英] Convert from UTC to local timezone give wrong result

问题描述

背景

我需要将格式为 HH：mm的时间字符串从UTC转换为本地时区。例如，如果UTC时间为09:00，则本地时间（斯德哥尔摩/欧洲）应提前两个小时。

Background
I need to convert time string with format: "HH:mm" from UTC to local timezone. For example if UTC time is 09:00, local time (Stockholm/Europe) should be two hours ahead.

问题

将09:00（UTC）转换为斯德哥尔摩/欧洲时间时，我会得到10:00。应该是11:00。

Problem
When I convert 09:00 (UTC) to Stockholm/Europe time I get 10:00. It should be 11:00.

func UTCToLocal(date:String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "HH:mm"
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")

    let dt = dateFormatter.date(from: date)
    dateFormatter.timeZone = TimeZone.current //Stockholm/Europe

    return dateFormatter.string(from: dt!)
}

print(UTCToLocal(date: "09:00")) //prints 10:00

为什么时区与预期的时区不同？

Why is the timezone different from what it suppose to be?

推荐答案

您仅指定了一个时间，但没有指定日期，因此 09:00将
转换为日期 2000-01-01 09:00:00 +0000。当时斯德哥尔摩的夏令时尚未启用，当地时间为10:00。

You specified only a time, but no day, therefore "09:00" is converted to the date "2000-01-01 09:00:00 +0000". At that moment daylight saving time was not active in Stockholm, and the local time was 10:00.

如果您希望当天进行转换，则可以设置

If you want the conversion for the current day then you can set

dateFormatter.defaultDate = Date()

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