feat: Add timestamp conversion for CACAOv3

[ukstv]

Rely on POSIX specification to convert between UTC and unix timestamps.

[oed]

Would be nice with some more detail here.

1. If we have timeRFC, how do we get timeUnix and timeTzAndMills?
2. If we have timeUnix and timeTzAndMills, how do we get timeRFC?

Basically how do we separate the one timestamp into the two fields and vice versa.

[chunningham]

instead of trying to split up the timestamp, could we instead just keep the whole timestamp in the fields? e.g.

fct: {
    iat-enc: "1985-04-12T23:20:50.52Z"
    exp-enc: "1990-12-31T15:59:60-08:00"
}

and require that the unix time of these matches those in iat, exp and nbf? it makes these fields slightly longer, but obviates the need for multiple fct fields per time field (i.e. instead of iat-tz and iat-mills we just have iat-enc), and also covers every base

[oed]

Whats the benefit of this though? We still need to do the conversion of the unix timestamp for verification.

I also never suggested two fct fields. would be more something like:

issued-at (RFC) turns into iat and fct.z-iat (the latter would contain both milliseconds and TZ info)

[chunningham]

ah I misunderstood, so we would represent everything after the seconds part? e.g.
1985-04-12T23:20:50.52Z => .52Z
1990-12-31T15:59:60-08:00 => -08:00
1990-12-31T15:59:60.52-08:00 => .52-08:00
In that case it seems to do the trick

[oed]

Yes exactly. And that's what I was hoping this appendix section would cover!

[ukstv]

I might miss or forget some discussion about timezones? Why does one need that? Also, milliseconds - are we in a high-frequency trading realm?

Unix timestamp is UTC (±1 second). You could always convert UTC to another timezone. A wallet definitely knows better how to display a timestamp in a user-friendly way, be it ISO8601 format or "20:00 next Monday".

Why do we have that even in CACAO spec instead of relying on a unix timestamp?

[oed]

I might miss or forget some discussion about timezones? Why does one need that?

SIWE supports the message having tz info

Also, milliseconds - are we in a high-frequency trading realm?

No but SIWE messages support it so we also need to in this conversion algo. We need to describe it precisely here.

[ukstv]

Oh shoot. Right.

[obstropolos]

cc @chunningham

[bumblefudge]

we should close #235 with this PR too (he says as if he's going to help)