Add support for java.time.Instant

[FlightOfStairs]

I see that this has previously been discussed and rejected: #42 #26

However, I don't believe that the reasoning is correct:

For Instant and LocalDateTime, the challenge is neither has an offset as @andrewsf said. It would be better for the user to choose the offset rather than the library to assume an offset.

I fully agree regarding LocalDateTime, but java.time.Instant does have an implicit offset of UTC:

For the segment from 1972-11-03 (exact boundary discussed below) until further notice, the consensus international time scale is UTC (with leap seconds). In this segment, the Java Time-Scale is identical to UTC-SLS. This is identical to UTC on days that do not have a leap second. On days that do have a leap second, the leap second is spread equally over the last 1000 seconds of the day, maintaining the appearance of exactly 86400 seconds per day.

https://docs.oracle.com/javase/8/docs/api/java/time/Instant.html

[ryber]

I agree, additionally, Instant is essentially the least common denominator of Date/Time classes. It's the easiest target to get to in a DTO object/record from whatever it is people are using in the backend.

Instant also both produces and parses ISO 8601 timestamps perfectly as long we you are happy with UTC (and who wouldn't be?)

        var now = Clock.systemUTC().instant();

        // outputs something like 2025-09-16T19:37:12.109887Z
        System.out.println("now = " + now.toString());

        var withoffset = Instant.parse("2023-10-15T10:15:30+02:00");

        // outputs 2023-10-15T08:15:30Z, note the time change properly reflected in UTC
        System.out.println("withoffset = " + withoffset.toString());

[andrewsf]

Right. My original PR comment was referenced to mean that Instant should not be supported because it has no offset, but I feel more that it has every offset, in that it bypasses the notion of an offset. A more precise version of SecondsSinceEpoch, and perhaps it should have been the first date/time scalar implemented.