Delphi: How to determine and empty TDatetime value

First you need to define what you mean by 'an empty TDateTime value'.

A TDateTime value is a double with the date encoded in the integer part and the time encoded in the fractional part. So, the closest thing to a 'null date' you can get is probably 0.

Hence, simply test ADate <> 0 to test if the date is 'null'.

But beware: if you declare a TDateTime local variable then it will not necessarily be =0 before you give it a value. It can be anything. Of course, the same thing applies to variables of type integer, double, boolean, ...

Also, I believe that a TDateTime with value 0 encodes the date 1899-12-30.

Finally, negative TDateTime values are perfectly normal. For instance, -5000 corresponds to 1886-04-22.

I don't quite get the point of your code. If you want to use 0 as the 'unassigned' value (which is bad if you are interested in dates close to 1899-12-30), why not do simply

function IsUnassigned(ADate: TDateTime): boolean;
begin
  result := ADate = 0;
end;

or, possibly (but not equivalently!),

function IsUnassigned(ADate: TDateTime): boolean;
begin
  result := IsZero(Date);
end;

In his answer, ain gave a couple of more reasonable choices for the 'unassigned date' value.

At unit Spring.Persistence.Core.Session.pas of library Spring Framework for Delphi (http://www.spring4d.org) in method TSession.ExecuteScalar<T> in case of NULL for result used value Default(T);

I think, Your function can look like

function IsNull(ADate: TDateTime): Boolean;
begin
  Result := ADate = Default(TDateTime);
end;

As Andreas already wrote, the TDateTime type is actually double and thus not "nullable". I use

const
  c_UnassignedDate = -693594;

for a empty date value as this represents an impossible date of 00/00/0000. But for example DevExpress uses

NullDate = -700000;
InvalidDate = NullDate + 1;

So there seems to be no agreed upon standard vale, you should pick one which suits your need.