Re: [Boost-users] Date-Time question on seconds since epoch

12 Jan 2005

      On Tue, 11 Jan 2005 09:22:06 -0500, Caleb Epstein wrote

Not sure what happened to my reply 12 hours ago, but I'll try again...
...
On Tue, 11 Jan 2005 07:26:20 -0600, Bill Lear  wrote:
...
I'd like to determine the total number of seconds between a date and
the epoch ('00:00:00 1970-01-01 UTC'), The date can be in a timezone
that has daylight savings.  The date should be converted to UTC, with
appropriate daylight savings adjustments made, and then the epoch
subtracted from it.  This should mirror the unix GNU 'date' utility,
which can be run as:
% TZ="America/New_York" date --date='2004-10-04 12:14:32' +%s
1096906472
I've looked at the doc, but it's not clear how to do this.  Could
someone give me a few pointers, please?
I would make use of the standard library facilities timelocal or
timegm to convert a struct tm to time_t format.  The former assumes
the time is expressed in local time (as specified by the TZ
environment variable or via tzset) and the latter assumes it is in
GMT.
The support for timezone conversions in Boost.DateTime is rudimentary
at best and does not currently make use of operating system 
facilities like the zoneinfo database.
Caleb is right, the released functionality in date-time makes this difficult
to do.  However, the current CVS actually has everything you need.  I wrote
the following small program that does the calculation using new features which
will be released as part of 1.33:

//convert.cpp
#include "boost/date_time/local_time/local_time.hpp"
#include <iostream>

int main()
{
  using namespace boost::gregorian; 
  using namespace boost::local_time;
  using namespace boost::posix_time;

  tz_database tz_db;
  tz_db.load_from_file("date_time_zonespec.csv");
  boost::shared_ptr nyc_tz = 
                      tz_db.time_zone_from_region("America/New_York");
  date in_date(2004,10,04);
  time_duration td(12,14,32);
  // construct with local time value -- create not-a-date-time 
  // if invalid (eg: in dst transition)
  local_date_time nyc_time(in_date, td, nyc_tz,                     
                           local_date_time::NOT_DATE_TIME_ON_ERROR);

  std::cout << nyc_time << std::endl;

  ptime time_t_epoch(date(1970,1,1)); 
  std::cout << time_t_epoch << std::endl;

  time_duration diff = nyc_time.utc_time() - time_t_epoch;
  std::cout << "Seconds diff: " << diff.total_seconds() << std::endl;
  return 0;
}

Output:
Mon Oct  4 12:14:32 2004 EDTEDT
1970-Jan-01 00:00:00
Seconds diff: 1096906472

I see there is a bug in the output code right now that prints the short time
zone abbreviation twice right now.  We are in the process of a major I/O
overall. But anyway, if you are willing to work with 'alpha' code with no docs
you can use it now.  I say alpha code, but there are already pretty complete
tests in the tests/local_time directory...

BTW, the tz_database class is reading a file you will find in cvs at
libs/date_time/data/date_time_zonespec.csv.  This CSV file contains a dump of
time zone database data into a form that is handing for reading, porting, and
editing.  This is what allows the regional time_zone specification that
contains all the dst rules, etc. 

HTH,

Jeff

Re: [Boost-users] Date-Time question on seconds since epoch

Jeff Garland