如何在Delphi中将本地时间转换成UTC时间?以及如何将其从UTC转换回当地时间? [英] How to convert Local time to UTC time in Delphi? and how to convert it back from UTC to local time?

问题描述

我正在使用Delphi，并尝试使用UTC日期时间在我的数据库中存储记录，然后在客户端在其本地日期时间中读取记录时将其还原回去吗?任何想法如何进行回溯转换吗?

I'm using Delphi and I'm trying to store records using UTC datetime in my database and then restore it back when a client reads it in his local datetime ? any idea how to do this forth back conversion ?

推荐答案

这是我用来从UTC转换为本地的函数.

This is the function that I use to convert from UTC to local.

function LocalDateTimeFromUTCDateTime(const UTCDateTime: TDateTime): TDateTime;
var
  LocalSystemTime: TSystemTime;
  UTCSystemTime: TSystemTime;
  LocalFileTime: TFileTime;
  UTCFileTime: TFileTime;
begin
  DateTimeToSystemTime(UTCDateTime, UTCSystemTime);
  SystemTimeToFileTime(UTCSystemTime, UTCFileTime);
  if FileTimeToLocalFileTime(UTCFileTime, LocalFileTime) 
  and FileTimeToSystemTime(LocalFileTime, LocalSystemTime) then begin
    Result := SystemTimeToDateTime(LocalSystemTime);
  end else begin
    Result := UTCDateTime;  // Default to UTC if any conversion function fails.
  end;
end;

如您所见，该函数按如下所示转换UTC日期时间:

As you can see the function transforms the UTC date time as follows:

• 日期时间->系统时间
• 系统时间->文件时间
• 文件时间->本地文件时间(这是从UTC到本地的转换)
• 本地文件时间->系统时间
• 系统时间->日期时间

很明显如何扭转这种状况.

It should be obvious how to reverse this.

请注意，此转换将夏令时视作现在是现在，而不是视其为/当时是.. DateUtils.TTimeZone 类型，已在XE，尝试做到这一点.代码变为:

Note that this conversion treats daylight saving as it is now rather than as it is/was at the time being converted. The DateUtils.TTimeZone type, introduced in XE, attempts to do just that. The code becomes:

LocalDateTime := TTimeZone.Local.ToLocalTime(UniversalDateTime);

从另一个方向使用 ToUniversalTime .

该类似乎(宽松地)基于.net TimeZone 类.

This class appears to be (loosely) modelled on the .net TimeZone class.

警告语.不要期望尝试将转换时的夏令时计算为100％准确.根本不可能做到这一点.至少没有时间机器.那只是考虑未来的时间.过去的时间很复杂.Raymond Chen在这里讨论此问题:为什么夏令时不直观.

A word of warning. Do not expect the attempt to account for daylight savings at the time being converted to be 100% accurate. It is simply impossible to achieve that. At least without a time machine. And that's just considering times in the future. Even times in the past are complex. Raymond Chen discusses the issue here: Why Daylight Savings Time is nonintuitive.

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