Avoid SimpleDateFormat

[tristantarrant]

Aside from the thread-safety issue (which in Xstream is circumvented via the use of thread-locals), SimpleDateFormat causes the JDK to initialize all possible timezones, resulting in a heap overhead of about 3.5MB (measured on x86_64).
It would be better to use DateTimeFormatter and avoiding the use of the z pattern (which also triggers the initialization of the timezones).

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("ddMMyyyy HH:mm z");
formatter.parse("01012023 00:00 CEST");

triggers the time-zone initialization, while

DateTimeFormatter formatter = DateTimeFormatter.ofPattern("ddMMyyyy HH:mm VV");
formatter.parse("01012023 00:00 Europe/Paris");

does not.
The following table summarizes the heap impact:

Strategy	LocalDateTime	LocalDate	ZoneInfo	ZoneOffset	Heap	Heap Delta (vs Baseline)
Baseline (no timezone calls)	180	0	0	0	1845638	0
Single Timezone [TimeZone.getTimeZone(id)]	180	0	0	0	1845936	298
All timezones with all variants [DateFormatSymbols.getZoneStrings()]	57076	32212	602	1455	5605744	3760106
All timezones with main variants [TimeZone.getAvailableIDs()+TimeZone.getTimeZone(id)+TimeZone.getName()]	36678	21674	632	1155	4870584	3024946
All timezones with no variants [TimeZone.getAvailableIDs()+TimeZone.getTimeZone(id)]	180	0	632	0	2298216	452578