#65 TryVar2DateTime returns False for a number and for a string that is not a date

[Thomas Mueller]

TryVar2DateTime in u_dzVariantUtils only handles a varDate, a string and custom variant types. Measured with Delphi 12:

Variant	Result
varDate 2026-10-14	True
String '2026-10-14'	True
Double 46309.5	False
Integer 46309	False
String 'abc'	False
Null, Empty	False

Two problems:

• A number is a valid TDateTime, and Var2DateTimeEx and Var2DateTimeStr accept it, but TryVar2DateTime returns False.
• For a value that cannot be converted, like the string 'abc', it returns False, while the other TryVar2Xxx functions raise EVariantConvertError (see [bugs:#63]).

Fix: accept everything that converts to TDateTime, return False for Null and Empty, raise EVariantConvertError for a value that cannot be converted, and give it the same optional _ErrorHandling: TErrorHandlingEnum = ehRaiseException parameter as the other TryVar2Xxx functions.

Note: this changes the behaviour for its callers, TNullableDate.AssignVariant, TNullableDateTime.AssignVariant and TryFieldAsDate in u_dzDatasetHelpers.

[Thomas Mueller]

Fixed in revision #2440