handle NULL TDates

[ghost]

Should the nil date be pretty as plain text or as date?
0 as plain Text would be 30.12.1899 as date
-693594 as plain text would be 00.00.0000 as date

[bvorak]

Is there any chance the user get's to see this date? It's about the return date right?
If so I'd opt for the second proposal.
Even though the GUI could handle those "not yet set" return dates and display them in an user friendly way.

[cnmicha]

nil just means "no date set", so e.g. for the return date, that the book was not returned yet, so replacing it by 0 or minus somewhat would lead to wrong values...

[ghost]

No the user should not see this date if we do everything rigth. Yes the return date.
Yes I also guess the second one is better.
Yes @cnmicha but nil is a Object from the type Pointer. The TDate type is an alias for the Double type that does not allow the nil pointer as valid value. So Lazarus gives a compile error back when we try to pass the nil pointer as TDate object.
When this is right I see no other way than these two options also it are "wrong values".

[cnmicha]

There is a "Null" value for doubles, I'll check if there is anything compareable for TDate..

[cnmicha]

There is not, so the database group will consider every date that is =-1, so =30.12.1899 as NULL value.

[cnmicha]

Please use uDBConstants and null check like this:
if TDateVar = SQLNull then
working with all integer/float like types
so the -1 value can be changed at one place anytime when neccessary

[ghost]

-693594 is the final value

[cnmicha]

I will apply to code...