Re: [Boost-users] Using boost::date_time for timezones

From: Jeff Garland (jeff_at_[hidden])
Date: 2004-08-18 16:25:19

On Wed, 18 Aug 2004 20:33:10 +0100, Duncan Woods wrote
> Hello All,
> 
> I'm new to Boost and interested in using it for timezone conversions 
> but I'm not finding it terribly clear. An example, which works fine, 
> uses something of the form:
> 
> typedef boost::date_time::local_adjustor<ptime, -5, us_dst> us_eastern;

Yes, this allows you to build static converters for a small number of time
zones with well defined rules.  If there isn't a rule set in the library that
suits your application you would need to create one of your own.  Since this
is totally undocumented, I'll help you if you really want to go this way.

> 1. My problem is that I wish to use DST settings other than US and 
> found definitions such as eu_dst_trait in local_timezone_defs.hpp 
> but these seem unused and unusable. Would it require implementation 
> of dst_rules.hpp type classes? If so, Boost would not give much advantage.

I agree date_time doesn't support local time adjustments well...yet.

> 2. I would also have liked to specify dst & timezone combinations at 
> run-time which dynamic_local_time_adjustor sounds like it might do - 
> but it appears to have no members.

It has members. Look at the bottom of
libs/date_time/test/posix_time/testlocal_adjustor.cpp.  Note the comment:
  //still experimental

> 3. Will date_time use the 'tz database' in the future?

The short answer is yes and I expect it before 1.33.  

The long answer is that in future releases there will be a generic time_zone
class that encapsulates the glob of interfaces that the library needs to
correctly handle a time zone.  The abstraction will use subclassing to support
different concrete time zone implementations.  We have a working prototype now
that uses Posix timezone strings to construct.  So you can do something like:

 posix_time_zone arizona_tz("MST-07");
 posix_time_zone ny_tz("EST-05EDT,M4.1.0,M10.5.0");

We are working on a variant that will use data from the TZ database so you can
say something like
 boost::shared_ptr<time_zone_base> tz(tz_database.find("America/New York"));

What we are NOT planning to do is supporting all the historic timezones of the
 timezone database -- doesn't seem like it's worth the complication.  We also
are not planning on reading the TZ database binary format directly (it's a
directory with binary files for each timezone for those that don't know). 
Rather we are looking at condensing the contents (probably fron VCALENDAR
form) down to a single csv file that can be read in by the application and can
be modified as need be using text editors, excel, etc.  So in your app it
would look something like:
 tz_database db("zones.csv");
 boost::shared_ptr<time_zone_base> tz(db.find("America/New York"));

Of course this comes with a new local_time class that can use time zones
natively to adjust, do calculations, input/output, etc.  So in particular, I
expect to be able to write the following kind of code
 tz_database db("zones.csv");
 boost::shared_ptr<time_zone_base> tz(db.find("America/New York"));
 local_date_time ny_time = second_clock::local_time(tz); 
 //2004-Aug-18 05:27:00 EDT 
 std::cout << "The current time in New York is: " << ny_time << std::endl; 

HTH,

Jeff