ISO8601Utils assumes local timezone while parsing "yyyy-MM-dd" dates

[roman-tikonov]

Parsing a "yyyy-MM-dd" date without a timezone using

ISO8601Utils.parse(dateString, ParsePosition(0)).toInstant()

applies a timezone from a default locale which leads to an incorrect result.

Input: 2019-03-20
Actual Output: 2019-03-19T23:00:00Z when a system default locale is UTC+01
Expected Output: 2019-03-20T00:00:00Z regardless of the system locale

I think the problem is that GregorianCalendar must be preconfigured with the UTC timezone, and it's not done in ISO8601Utils:

// if the value has no time component (and no time zone), we are done
            boolean hasT = checkOffset(date, offset, 'T');
            
            if (!hasT && (date.length() <= offset)) {
                Calendar calendar = new GregorianCalendar(year, month - 1, day);

                pos.setIndex(offset);
                return calendar.getTime();
            }

[pavlo-zin]

Any updates on this?

[eamonnmcmanus]

It's certainly regrettable that the code is the way it is, but as I mentioned in #2480 we can't change it without breaking existing deployments. It's an internal class, so really I think what we're talking about is its use in DateTypeAdapter and DefaultDateTypeAdapter. I think we should probably have a configuration option that causes Date to be serialized following ISO 8601, and that might also imply that a deserialized date/time without a timezone defaults to UTC.