datetime - How to convert Local time to UTC time in Delphi? and how to convert it back from UTC to local time?

Question

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 ?

Answer 1

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;

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

• Date time -> system time
• System time -> file time
• File time -> local file time (this is the conversion from UTC to local)
• Local file time -> system time
• System time -> date time

It should be obvious how to reverse this.

---

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);

In the other direction use ToUniversalTime.

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

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.