[Cbor] Multiple ways of expressing the same time via draft-bormann-cbor-time-tag

[Emile Cormier <[e-mail]>]

I'm implementing tags 1001 and 1002 in my C++ library to exchange time
points and durations (refer to
https://www.ietf.org/archive/id/draft-bormann-cbor-time-tag-04.html).

I'm in the situation where I''m not sure which is the "expected" way to
encode time as nanoseconds since the Unix epoch. I'm specifically referring
to std::chrono::system_clock::time_point, which is a 64 bit integer. There
are currently three ways I could encode such a value.

1. The whole number of seconds under Key 1, with the remaining nanoseconds
under Key -9.

2. The total number of nanoseconds under Key -9, with a value of zero under
Key 1.

3. The total number of nanoseconds expressed as a Decimal Fraction under
Key 4.

Option #2 is the easiest and most efficient because it doesn't involve a
division/subtraction calculation. However, a receiver that discards Key -9
will end up with zero seconds. At least with option #1, the receiver will
know the number of whole seconds if Key -9 is ignored.

Option #3 requires that the sender and receiver understand Tag 4 (Decimal
Fraction), or at least the concept of decimal fractions within the context
of tags 1001-1003.

I'm guessing that option #1 is the "canonical" way, but I'm not entirely
sure. Some guidance would be appreciated. Perhaps a few words concerning
this should be added to the standard's text?

On the receiving side, my library would ideally handle all three scenarios.
It's on the sending side I'm more concerned about when I need to
interoperate with CBOR decoders other than mine.

I apologize if this has already been discussed before. I'm a bit late to
the CBOR party. :-)

Cheers,
Emile Cormier