Default value for date/time

[Stephen Colebourne]

There is an implicit desire in adding value types that classes like
`LocalDate` can be treated as value types. But one of the requirements
of value types is that they have a default value. But what should this
be?

For `LocalDate`, possible candidates might be

- LocalDate.of(1970, 1, 1)
- LocalDate.of(1, 1, 1)
- LocalDate.of(2000, 1, 1)
- LocalDate.MIN
- LocalDate.MAX
- LocalDate.now()

But these are all poor choices IMO. Ultimately, dates are not like
numbers - there isn't a meaningful default like zero.

I suppose we'll have to use 1970-01-01 as it is traditional in Java,
but it doesn't really have *meaning*. I'd be interested in hearing if
there are any other choices or views?

(I think this problem of no sensible default value may well occur for
other value types.)

Stephen