How know type variable is TDateTime, TDate and TTime in Delphi

Question

I need know type variable TDateTime, TDate and TTime.

Anyone have any idea how to do this?

I used the code below, the result is "Is NOT TDateTime", "Is NOT TDate", "Is NOT Ttime"

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.Rtti,
  System.SysUtils;

var
  DateTime, Date,Time: TValue;

begin

  DateTime:= StrToDateTime( '01/01/2013 01:05:09' );
  if ( DateTime.TypeInfo = System.TypeInfo(TDateTime) ) then
    Writeln( 'Is TDateTime' )
  else
    Writeln( 'Is NOT TDateTime' );

  Date:=  StrToDate( '01/01/2015' );
  if ( Date.TypeInfo = System.TypeInfo(TDate) ) then
    Writeln( 'Is TDate' )
  else
    Writeln( 'Is NOT TDate' );

 Time:=  StrToTime( '01:01:02' );
  if ( Date.TypeInfo = System.TypeInfo(TTime) ) then
    Writeln( 'Is TTime' )
  else
    Writeln( 'Is NOT TTime' );

 Readln;

end.

Thanks

Answer 1

The Implicit operator overloads of TValue got you.

When you assign the result of StrToDateTime, StrToDate and StrToTime to a TValue it uses the most matching Implicit operator overload from TValue which is Extended.

Also keep in mind that all three functions return TDateTime so even if there were operator overloads for TDateTime, TDate and TTime it would not work as expected.

To get the correct results you would have to explicitly specify the type when assigning your values to the TValue variables:

DateTime := TValue.From<TDateTime>(StrToDateTime( '01.01.2013 01:05:09' ));

Date:= TValue.From<TDate>(StrToDate( '01.01.2015' ));

Time:= TValue.From<TTime>(StrToTime( '01:01:02' ));

Answer 2

Just in case you were trying to determine the result type of StrToDateTime:

type
  TDateType = (dtDate, dtDateTime, dtTime);

function getDateType(date: TDateTime): TDateType;
begin
  if Trunc(date) = date then // Or DateOf(date), if available
  begin
    Result := dtDate;
  end
  else
  begin
    if Trunc(date) = 0 then // Or DateOf(date), if avaialble
    begin
      Result := dtTime
    end
    else
    begin
      Result := dtDateTime;
    end;
  end;
end;

// Sample
var
  result: TDateType;
begin
  result := getDateType(StrToDateTime('01/01/2013 01:05:09')); // dtDateTime
  result := getDateType(StrToDateTime('01/01/2015')); // dtDate
  result := getDateType(StrToDateTime('01:01:02')); // dtTime
  // One caveat
  result := getDateType(StrToDateTime('01/01/2013 00:00:00')); // dtDate
end;

Alternatively, you could use the TryStrToDate, TryStrToTime, and TryStrToDateTime functions.

Answer 3

In case you're curious, the TDateTime is encoded as floating point Double internally.

TDateTime internals
The fractional part denotes the time, the integer part denotes the date.
Knowing this the following tests will evaluate to true.

dtTime: ABS(Double(DateTime1)) < 1.0 
dtDate: Trunc(Double(DateTime1)) = Double(DateTime1)
dtDateTime:  (     (ABS(Double(DateTime1)) > 1.0) 
         and (Trunc(Double(DateTime1)) <> Double(DateTime1)) )

Obviously that's a very roundabout way of testing, but sometimes it helps to know how a TDateTime is shaped internally.

DateUtils
The sane versions of these tests would be:

uses DateUtils;

dtDate: DateTime1 = DateOf(DateTime1)
dtTime: DateTime1 = TimeOf(DateTime1)
dtDateTime:(DateTime1 <> DateOf(DateTime1)) and (DateTime1 <> TimeOf(DateTime1))

TDateTime is compatible with Excel
The value 0 links to the epoch in Microsoft's Excel: 30 December 1899; 12:00 A.M.
^{(It should have been 1-1-1900, but they changed it to compensate for an error in Lotus' 123 date algorithm)}
This is great, because Delphi's TDateTime is fully compatible with Excel's DateTime.

Here's the official doc: http://docwiki.embarcadero.com/Libraries/XE5//en/System.TDateTime