Question

I’m trying to convert the value returned by Now() to double and then use it in a different function, ie.

procedure TForm1.FormCreate(Sender: TObject);
begin
    MyFunction_1(Now);
end;

procedure MyFunction_1(TheTime : Double);
begin
    MyFunction_2(TheTime);
end;

procedure MyFunction_1(ModificationDate : Double);
begin
    if JulianDateToDateTime(ModificationDate) < 12345 then
       ...
end;

But the problem is, the current time value isn’t passed correctly. A simple test for JulianDateToDateTime(Now); returned something like this:

0/0/0000 16:10:16

I’m pretty sure it’s something really basic, I just can’t figure out what’s it is

Any help please?!!

Answer 1

You’re doing it backward. 🙂

JulianDateToDateTime takes a Julian date as a parameter and returns a TDateTime. It does’t take a TDateTime as an argument; that’s why you’re getting the wrong result. I think you mean to go the other way (DateTimeToJulianDate).

From the Delphi documenation:

Call JulianDateToDateTime to convert a Julian date to the corresponding TDateTime value

Call DateTimeToJulianDate to convert a TDateTime value into the corresponding Julian date.

Here’s a quick sample showing the difference:

uses DateUtils;

procedure TForm2.FormShow(Sender: TObject);
var
  TodaysDate: TDateTime;
  JDate: TDateTime;
begin
  TodaysDate := Date;
  JDate := DateTimeToJulianDate(TodaysDate);
  ShowMessage('Today: ' + FloatToStr(TodaysDate) + #13 +
              'JDate: ' + FloatToStr(JDate) );
end;

Here’s the output:

Note that adding one more variable of TDateTime type and trying to use JulianDateToDateTime raises a range exception:

BadDate: TDateTime;
...

BadDate := JulianDateToDateTime(TodaysDate);   // raises exception