[fpc-devel] Definition of the time-fration in a TDateTime before 30/12/1899

[Michael Van Canneyt]

On Sun, 21 May 2006, Joost van der Sluis wrote:

> On Sun, 2006-05-21 at 16:40 +0200, Michael Van Canneyt wrote:
> > On Sun, 21 May 2006, Joost van der Sluis wrote:
> > > I have some questions about the TDateTime format.
> > >
> > > I always thought that you could do this:
> > >   TD : TDateTime
> > >
> > > TD := EncodeDate()+encodeTime();
> > >
> > > This would mean that you have the following: (24-hours notation)
> > >
> > >  0     = 12/30/1899 00:00    (as defined)
> > >  1     = 12/31/1899 00:00
> > >  1.25  = 12/31/1899 06:00
> > > -1     = 12/29/1899 00:00
> > > -2     = 12/28/1899 00:00
> > > -1.25  = 12/28/1899 18:00    (-2+0.75)
> > >
> > > But now the Delphi help says:
> > >
> > > -1.25  = 12/29/1899 06:00
> > 
> > It's also what Delphi does.
> > 
> > >
> > > And this is how it's also implemented in FPC. But for instance
> > > PostgreSQL which also uses a format like TDateTime does it the other
> > > way. Which looks more logical to me, since if you want to know if
> > > DateTimeA is after DateTimeB you can do: DateTimeA>DateTimeB
> > >
> > > With the Delphi-help variant this won't work. Since -1>-1.25 but
> > > 12/29/1899 00:00 > 12/29/1899 06:00 doesn't make sence to me.
> > >
> > > This implementation will mean that if you compare TDateTime's you first
> > > have to check whether the date is before or after 12/30/1899 ?
> > >
> > > There are some bugs in DecodeDate, DecodeTime and friends for dates
> > > before 12/30/1899 but to fix those, I need to know what to do with this
> > > problem.
> > 
> > Keep delphi compatibility.
> > 
> > Also windows compatibility. TDateTime is a windows construct.
> > (see variants)
> > 
> > They obviously judged that the idea of
> >  TD := EncodeDate()+encodeTime();
> > was more important than strict ordering.
> 
> That also doesn't work. If td < 0 you must do: 
>   TD := EncodeDate()+(1-EncodeTime());

That is not correct, AFAIK ?

> 
> > The following remark in the Delphi help sheds more light on the issue:
> > 
> > "When working with negative TDateTime values, computations must handle
> > time portion separately. The fractional part reflects the fraction of
> > a 24-hour day without regard to the sign of the TDateTime value.
> > For example,
> >   6:00 am on 12/29/1899 is -1.25,
> > Not
> >   -1 + 0.25,
> > which would be -0.75. There are no TDateTime values between -1 and 0."
> 
> That explains why -0.5 is the same as 0.5.
> Where in the help can I find that?

TDateTime type help.

Michael.