#68 Var2DateTimeStr/Var2DateStr/Var2DateStrEx convert strings, Booleans and numbers differently from TryVar2DateTime

[Thomas Mueller]

The date functions in u_dzVariantUtils do not agree on what can be converted. TryVar2DateTime (and Var2DateTimeEx, which is built on it, see [bugs:#65]) converts a string with TryStr2DateTime (Windows format, German format, ISO 8601), rejects a Boolean, and accepts a number only between 0001-01-01 and MaxDateTime. The others do not:

• Var2DateTimeStr and Var2DateStr convert a string with the RTL's variant cast, which only knows the Windows format (plus a number in a string). So they reject the ISO format they produce themselves: a round trip through them fails.
• Var2DateStrEx uses Str2Date, which raises EConvertError instead of EVariantConvertError, and has no range check, so FormatDateTime raises for an out-of-range number.
• All three convert a Boolean to 1899-12-29 (True) or 1899-12-30 (False).

Measured with Delphi 12 and 2007, short date format dd/mm/yyyy:

Variant	TryVar2DateTime	Var2DateTimeStr / Var2DateStr	Var2DateStrEx
'2026-10-14'	True	EVariantConvertError	2026-10-14
'14.10.2026'	True	EVariantConvertError	2026-10-14
'14.10.2026 13:45:00'	True	EVariantConvertError	EConvertError
'46309.5'	False	2026-10-14 12:00:00 / 2026-10-14	EConvertError
'abc'	False	EVariantConvertError	EConvertError
True	False	1899-12-29	1899-12-29
1e20	False	EVariantConvertError	EIntOverflow (12), EInvalidOp (2007)

Fix: build Var2DateTimeStr, Var2DateStr and Var2DateStrEx on TryVar2DateTime(..., ehNoException), so all date functions convert the same way and raise EVariantConvertError for a value that cannot be converted. This changes which strings Var2DateTimeStr and Var2DateStr accept: a number in a string would no longer be accepted.

Found by a code review of the fix for [bugs:#65].

[Thomas Mueller]

Fixed in revision #2443