Delphi – How does Delphi calculate TDateTime as a Float value

Ok, my reaction is a bit late, but here's a solution for the newer Delphi's.

It uses implicit class overloaders so that records of this type can be used as if they are TDateTime variables.

  TDateRec = record
    year,month,day,hour,minute,second,millisecond:word;
    class operator implicit(aDateRec:TDateRec):TDateTime;
    class operator implicit(aDateTime:TDateTime):TDateRec; // not needed
    class operator implicit(aDateRec:TDateRec):String; // not needed
    class operator implicit(aDateRec:String):TDateRec; // not needed
  end;

Implementation:

uses DateUtils;

class operator TDateRec.Implicit(aDateRec:TDateRec):TDateTime;
begin
  with aDateRec do // Yeah that's right you wankers. I like "with" :)
    Result := encodeDateTime(Year,Month,Day,Hour,Minute,Second,Millisecond);
end;

class operator TDateRec.Implicit(aDateTime:TDateTime):TDateRec;
begin
  with Result do
    DecodeDateTime(aDateTime,Year,Month,Day,Hour,Minute,Second,Millisecond);
end;

class operator TDateRec.Implicit(aDateRec:TDateRec):String;
begin
  Result := DateTimeToStr(aDateRec)
end;

class operator TDateRec.Implicit(aDateRec:String):TDateRec;
begin
  Result := StrToDateTime(aDateRec)
end;

Now you can declare your dates like this:

const
  Date1:TDateRec=(Year:2009;month:05;day:11);
  Date2:TDateRec=(Year:2009;month:05;day:11;hour:05);
  Date3:TDateRec=(Year:2009;month:05;day:11;hour:05;minute:00);

To see if it works, execute the following:

ShowMessage(Date1); // it can act like a string
ShowMessage(DateToStr(Date1)); // it can act like a date

If you really want to replace all your TdateTime variables with this, you probably need to overload some other operators too (Add, subtract, explicit, ...).

"As usual" ;-) I'll point to the DSiWin32 which includes function DSiGetFileTimes which returns creation time, last access time and last modification time.

function DSiFileTimeToDateTime(fileTime: TFileTime; var dateTime: TDateTime): boolean;
var
  sysTime: TSystemTime;
begin
  Result := FileTimeToSystemTime(fileTime, sysTime);
  if Result then
    dateTime := SystemTimeToDateTime(sysTime);
end; { DSiFileTimeToDateTime }

function  DSiGetFileTimes(const fileName: string; var creationTime, lastAccessTime,
  lastModificationTime: TDateTime): boolean;
var
  fileHandle            : cardinal;
  fsCreationTime        : TFileTime;
  fsLastAccessTime      : TFileTime;
  fsLastModificationTime: TFileTime;
begin
  Result := false;
  fileHandle := CreateFile(PChar(fileName), GENERIC_READ, FILE_SHARE_READ, nil,
    OPEN_EXISTING, 0, 0);
  if fileHandle <> INVALID_HANDLE_VALUE then try
    Result :=
      GetFileTime(fileHandle, @fsCreationTime, @fsLastAccessTime,
         @fsLastModificationTime) and
      DSiFileTimeToDateTime(fsCreationTime, creationTime) and
      DSiFileTimeToDateTime(fsLastAccessTime, lastAccessTime) and
      DSiFileTimeToDateTime(fsLastModificationTime, lastModificationTime);
  finally
    CloseHandle(fileHandle);
  end;
end; { DSiGetFileTimes }