I like José’s proposal for {RD/JD, milliseconds} a lot.
I wonder if we can cater for the leap second issue on conversion to this intermediate format. It seems easier and more intuitive to convert to milliseconds elapse including leap second offset at this stage. Then the calculation of date/time difference is simpler - albeit still have to watch out for a negative offset and adjust accordingly.
As far as I can tell we just need the leap_seconds to apply the relevant offset. It would also make for easier pattern matching for far forward and far past dates since leap seconds have a short history (and maybe a short future!).
If this becomes the agreed approach then is it better to separate out Calendar from core so that it can be updated in cycle with leap year announcements and out of cycle with Elixir core?
As for intra-day conversions - my understanding is that the leap second is always in inserted/removed at 23:59:59 UTC. Therefore there still remains a challenge of conversion for non-UTC times and therefore the time difference for non-UTC times/datetimes is somewhat problematic?
