Java 8 LocalDate/Time conversion

[hatsuyuki15]

Hi, currently the LocalDateConverter is using systemDefault() as the zone id for converting from Date to LocalDate and vice verse.

This behaviour, however, will create a dependency on the OS timezone and thus, contradicts to the concept of LocalDate (date-time object without timezone). It also leads to inconsistency if the code are executed on multiple systems with different timezones, for example, persisting a LocalDate 2018-12-02 will result as 2018-12-01 17:00:00.000Z on UTC+7, and 2018-12-02 00:00:00.000Z on UTC.

I suggest using UTC timezone instead of systemDefault() one for java 8 LocalDate/Time conversion.

[tbroyer]

May I suggest storing them as strings? (or possibly numbers to take less storage space, e.g. Number(20181202) for a 2018-12-02 local date)

[evanchooly]

Changing the on disk representation would break anyone with existing data. The "nicest" way to change that would be to use a flag on MapperOptions, say, or a different Converter to be a configured. Neither of those options are terribly appealing at the moment.

[evanchooly]

OK. I've come up with a plan, finally, I'm ok with. Here's what I'm thinking:

1. Add an option to determine the timezone to MapperOptions: SYSTEM_DEFAULT and UTC
2. The default value will be SYSTEM_DEFAULT.
3. SYSTEM_DEFAULT will be deprecated in 1.5.0 and Morphia will log a warning when it is used suggesting a data migration to use UTC instead
4. In 2.0, the option will be removed altogether and Morphia will only use UTC when storing dates/times.

How does that sound to you? I like that it course corrects an admittedly bad design without immediately breaking any existing databases.

[hatsuyuki15]

That sounds pretty good to me!

[evanchooly]

If you could look over the commit above and check my sanity, but that seems to be what you need. Dates are easy to mess up so I'd love another set of eyes to double check me.