[boost] [date_time] Time zone improvements

Subject: [boost] [date_time] Time zone improvements
From: Rutger ter Borg (rutger_at_[hidden])
Date: 2008-10-20 09:08:42

Dear Boost.Date_Time developers,

Being a heavy user of Boost's Date_Time library, I think it is very useful.
However, I think one area could use improvement: its time zone handling.

I would like to bring the full Olson tz database [1] to Boost.Date_Time.
This offers a very high-quality, and, on package-managed-UNIX-systems, a
maintainance-free solution to a time zone database. I have written a modern
C++ implementation for reading zic-compiled binary zoneinfo-files.

At the moment, Boost.Date_Time is not able to represent time zones that
have DST offsets and/or zone abbreviations depending on (or, changing over)
time. I.e., if a country or locality changes its time zone definitions.
E.g., a system that is in "Europe/Amsterdam" has changing DST offset and
time zone
abbreviation, which depend on time

$ date -d "2008-10-10 20:00:00Z"
Fri Oct 10 22:00:00 CEST 2008
$ date -d "1930-10-10 20:00:00Z"
Fri Oct 10 20:19:32 AMT 1930

I would like to propose changes that enables Boost.Date_Time to represent
time zone definitions that change over time. The semantics will remain
similar to the current version,

boost::shared_ptr< time_zone >  my_zone( "Europe/Amsterdam" );
local_date_time some_time( pt, my_zone );

some_time.utc_time();    // 2008-Oct-10 20:00:00
some_time.local_time();  // 2008-Oct-10 22:00:00
some_time.is_dst();      // true
some_time.zone_abbrev(); // "CEST"
some_time.dst_offset();  // 02:00:00

in addition, the abstract Time Zone interface and supported accessors will
have to undergo a change. Although the desired result may be achieved in
multiple ways, I think the time zone base could support accessors like
from_utc( ptime const& p) and from_local(...) methods, perhaps returning a
(pointer to a) ttinfo tuple,

// UTC offset, is_dst, abbreviation
typedef boost::tuple< time_duration_type, bool_type, string_type > ttinfo; 

and/or a special type in case the conversion is ambigious. When using the
Olsen data base, this structure could be looked up through from_utc using
std::lower_bound on a std::map or a ptr_map of transition points. 

I see a local_time_iterator is absent from the library, which I would like
to add as well. 

By way of this email I would like to ask whether there is interest the
proposed improvement. If so, please let me know, so we can work on the
details.

Kind regards,

Rutger ter Borg

[1] Sources for Time Zone and Daylight Saving Time Data,
http://www.twinsun.com/tz/tz-link.htm