[tz] User time zones

[Eric Muller]

>   always transmit and store times
> in UTC.

I don't think this is perfect.

The issue is that the conversion between local and UTC involves the tz 
database (or similar) and that is a moving target. You can very well 
have tz_local_to_utc (t0, tzid, tzversion1) != tz_local_to_utc (t0, 
tzid, tzversion2), (after all, we would not need many versions of tz 
every year if that was not the case) and therefore tz_utc_to_local 
(tz_local_to_utc (t0, tzid, tzversion1), tzid, tzversion2) != t0.

A possible scenario: create an appointment for a time in the future 
using tzversion1, store the UTC time, and later read the appointment 
using tzversion2; the time of the appointment may have changed. It is 
true that in many parts of the world, the likelihood of that scenario is 
low, but there are others where it's far less uncommon. Also note that 
the scenario applies equally well to client/server vs. local, one user 
creating and another reading vs. the same user doing both.

Storing both local and UTC may not be the best to fix this, or not quite 
enough, because it does not tell you which of the two values is the 
authoritative one.

I don't quite see any other solution than storing a time together with 
the timezone in which it is expressed. That time zone may be UTC. If you 
make an appointment for a meeting in Moscow, then you really want to 
store the local time (it does not matter much how that local time 
relates to UTC, you'd better be there when the local clocks say so); if 
you make an appointment for a teleconference with participants across 
the world, then you really want to store the UTC time, and let everyone 
figure out what it mean (or will mean) for them. That puts a burden on 
users of choosing the timezone, but fortunately, local time is a good 
default, and only those who deal with multiple time zones have to choose 
UTC.

There is the subsidiary issue that what people really need is "the 
timezone of this location", rather than "this tz timezone". The 
difference shows up in the tz database whenever a new tz timezone is 
introduced. Fortunately, such changes are much less frequent.

Eric.