time-zone-related comments on draft-ietf-calsch-sch-00.txt
Paul Eggert
eggert at twinsun.com
Fri Jan 3 02:44:38 UTC 1997
Here are some comments on the time-zone-related parts of your Internet Draft
``MIME application/calendar Content Type''
<URL:ftp://ds.internic.net/internet-drafts/draft-ietf-calsch-sch-00.txt>
(November 1996). Please let me know if you have any questions about
these comments, and I'd appreciate hearing about any updates to your draft.
The comments below are similar to comments I've made about
draft-ietf-calsch-csct-00.txt and draft-newman-datetime-00.txt, which
I can forward if you're interested.
3.2.3.19 TZID
This section suggests that it is ``likely that we want an IANA
registry of time zone/daylight rule names''. I suggest that
you consult the public domain time zone database maintained by
Olson et al (<URL:ftp://elsie.nci.nih.gov/pub/>, updated
regularly) for ideas about how such a registry should be
maintained. The Olson scheme is an upward-compatible extension
of the Posix TZ string representation for time zone and
daylight saving rules. For example, in the Olson scheme, the
label `America/Los_Angeles' identifies the complete UTC offset
history for Los Angeles from to the introduction of standard
time in 1883 to the indefinite future. The Olson database is
updated as governments change the rules, so the label
`America/Los_Angeles' can be used by application programs
without worrying about the details.
It should be easy to convert the Olson database to any proposed
IANA registry of time zone and daylight saving info. Perhaps,
as a first cut, you could start with the Olson database as the
IANA registry. You might contact the mailing list
tz at elsie.nci.nih.gov if you are interested in using their work.
3.2.3.20 TZBias
3.2.3.21 TZDstBias
3.2.3.22 TZStdBias
3.2.3.23 TZStdTrans
3.2.3.24 TZDstTrans
3.3.1.2 Definitions
3.3.1.4 Recurring Event Date and Time Specification
The underlying model in these sections is too simplistic. It
assumes that there is a ``standard time zone'' for the home
calendar system, together with a single date and time for
entering and exiting daylight saving time. However, locations
sometimes alter the UTC offset of standard time (e.g. Portugal
and Sri Lanka in 1996). Also, some locales have observed
double daylight saving time for part of the summer (e.g. Britain
in 1947), and TZStdBias does not suffice to represent this.
The model should be fixed to allow an arbitrary sequence of UTC
offset changes. If necessary, each change could be labeled by
a daylight saving flag (to say whether the resulting time is
considered to be ``daylight saving''), and a string label
(e.g. `EST') that can be internationalized (e.g. to `HNE' for
French-speakers in Montreal). This would be simpler and more
accurate than the model in draft-ietf-calsch-sch-00.txt.
I suggest that you consult the Olson database for ideas about
how time zone and daylight saving information could be represented.
3.3.1.2 Definitions
The proper term is ``daylight saving time'', not ``daylight
savings time'' (at least, that is what the OED and the Random
House Unabridged Dictionary prefer).
``Pacific Daylight Savings Time'' should be ``Pacific Daylight Time''.
3.3.1.4 Recurring Event Date and Time Specification
This section does not address the problem of ``impossible
times'' (e.g. `1996-04-07 02:30:00', which does not denote a
valid local time in Los Angeles) or ``ambiguous times'' (e.g.
`1996-10-27 01:30:00', which denotes two distinct valid local
times in Los Angeles). When a time is converted from some
locale to UTC, it may be found to be impossible or ambiguous;
in that case, what should a scheduling application do?
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