Delphi: How to determine and empty TDatetime value

Question

Seems there is no way to assign NULL (either an "unassigned value" to TDateTime variables.

The only way I've imagined is using something like this:

function isNull(aDate : TDateTime) : boolean;
const NullDate = 0.0;
var aNullDate : TDatetime;
    ms : Int64;
begin
  aNullDate := NullDate;
  ms := MilliSecondsBetween(aDate,aNullDate);
  result := (ms = Int64(0));
end;

Is there anybody out who knows better solution what not overlaps 0 date value?

Are negative TDateTime values dangerous? (As an able resource for previous purpose)

If you intend to use this function the was I think, surely `NullDate` should be a global constant? — Andreas Rejbrand, Feb 20 '13 at 16:59
I've experienced problems when comparing a zero assigned value TDateTime variable to the const value 0. A _simple_ **if** as this one didn't check the condition ` var v_datetime: TDatetime;` ` v_datetime := 0;` ` if (v_datetime = 0) then actionToTake` — xsubira, Feb 20 '13 at 17:30
0 is a perfectly valid date, and negative dates are perfectly fine as well. 0 is midnight on 30 Dec 1899. Negative numbers are dates before that. — Nick Hodges, Feb 20 '13 at 21:03

ain · Accepted Answer · 2013-02-20T17:06:25.307

20

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.

edited Feb 20 '13 at 17:06

answered Feb 20 '13 at 16:44

ain

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2

There is no date where **day** and **month** are 0 (zero). In database, if the date is unassigned then `NULL` should be used, not some magic date. – ain Feb 20 '13 at 16:51
1

Yeah, sorry. Taking back, I was looking just at the year portion of that date... Well, if the value of a certain date time column will be `NULL` and I would have to store this value strictly to a `TDateTime` field, I would probably use your `c_UnassignedDate` constant value. [+1] – TLama Feb 20 '13 at 17:00
How do you figure that -693594 represents the date 0/0/0000? – Rob Kennedy Feb 20 '13 at 18:05
1

@Rob: I guess because `-693591` represents `0000-01-03`, `-693592` represents `0000-01-02`, and `-693593` represents `0000-01-01`, at least according to `DateTimeToStr`. – Andreas Rejbrand Feb 20 '13 at 18:16
@RobKennedy just do `ShowMessage(FormatDateTime('dd.mm.yyyy',-693594));` and it pops up dlg with `00.00.0000`. – ain Feb 20 '13 at 19:47
@ain: As does every smaller value. My analysis shows that -693594 is the greatest negative integer with this property. – Andreas Rejbrand Feb 20 '13 at 19:52

Andreas Rejbrand · Answer 2 · 2013-02-20T16:55:58.380

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.

Negative time dates become meaningless eventually, when the calendar breaks. — David Heffernan, Feb 20 '13 at 16:50
@David: Good point. So there are actually plenty of plausible choices for the 'unsassigned' value. — Andreas Rejbrand, Feb 20 '13 at 16:51
Would it not be useful to use NaN as an explicit value here? You can then use IsNaN to test for the condition. — Brian Frost, Feb 21 '13 at 07:52

score 3 · Answer 3 · answered Jan 23 '20 at 10:59

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;

score 1 · Answer 4 · edited Jan 02 '17 at 08:53

1

Use PDateTime instead TDateTime, and send as nil value. If no pointer to value, result no value.

To check value use Assigned ptr :

MyDatePointer := nil;
if ( Assigned(MyDatePointer)) then ...

edited Jan 02 '17 at 08:53

Sнаđошƒаӽ

16,753
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answered Jul 24 '15 at 12:03

Eugene.LatDev

11
1

score 1 · Answer 5 · answered Feb 03 '16 at 14:44

1

tDateTime is undefined for the values between 0 and -1 which means that a tDateTime of -0.5 is an undefined value and useful for a null date as an alternative to NaN. DateTimeToString will display -0.5 the same as +0.5;

answered Feb 03 '16 at 14:44

John Hillier

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Delphi: How to determine and empty TDatetime value

5 Answers5