From pic at superfluo.org  Sun Jan  7 17:17:41 2007
From: pic at superfluo.org (Nicola Piccinini)
Date: Sun, 07 Jan 2007 23:17:41 +0100
Subject: [TZInfo-users] timezones, periods and utc_offset
Message-ID: <45A17185.3090702@superfluo.org>

Hi all,

sorry for the silly question:
is the result of
tz.current_period.offset.utc_offset
constant over time?
In other words: is it independent from the current period? (as I suppose)

Thanks.
-- 
Nicola Piccinini -- http://superfluo.org


From phil.ross at gmail.com  Sun Jan  7 19:00:48 2007
From: phil.ross at gmail.com (Philip Ross)
Date: Mon, 8 Jan 2007 00:00:48 +0000
Subject: [TZInfo-users] timezones, periods and utc_offset
In-Reply-To: <45A17185.3090702@superfluo.org>
References: <45A17185.3090702@superfluo.org>
Message-ID: <4ac540410701071600p63619230w4f0eadadae122f2@mail.gmail.com>

On 07/01/07, Nicola Piccinini <pic at superfluo.org> wrote:
> sorry for the silly question:
> is the result of
> tz.current_period.offset.utc_offset
> constant over time?
> In other words: is it independent from the current period? (as I suppose)

I don't think its a silly question - the documentation could do with
some improvements.

utc_offset is the base offset from UTC.  For example, Europe/Rome is
at UTC + 1 hour and America/New_York is at UTC - 5 hours.  std_offset
is the offset to the usually observed time as given by utc_offset.  It
will typically vary between 0 and +1 hour according to daylight
savings. utc_total_offset is utc_offset + std_offset.

utc_offset tends to stay constant over time.  It won't necessarily
remain the same across all periods though because countries sometimes
redefine their timezones.

It's also worth noting that you should be using
tz.current_period.utc_offset rather than
tz.current_period.offset.utc_offset. start_transition, end_transition
and offset shouldn't really be publicly available and may be removed
in a future release.

-- 
Phil Ross
http://tzinfo.rubyforge.org/ -- DST-aware timezone library for Ruby

From brandon at opensoul.org  Tue Jan  9 00:50:12 2007
From: brandon at opensoul.org (Brandon Keepers)
Date: Tue, 9 Jan 2007 00:50:12 -0500
Subject: [TZInfo-users] using TZInfo to generate iCalendar VTIMEZONE
Message-ID: <E1670EBA-7379-4166-9247-786937C3A0CD@opensoul.org>

I'm working on a CalDAV client for Ruby, and part of the icalendar  
specification requires that you define any timezones that you you  
use.  From what I've gathered, this timezone definition should  
include all the rules for dates in the icalendar file.  So, if I have  
dates spanning from Jan 2006 - June 2007, then I would need to  
include 4 rules, the old standard and daylight times, and the new  
standard and daylight times.  Here is an example of the timezone  
definition:

BEGIN:VCALENDAR
PRODID:-//Example Corp.//CalDAV Client//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:US-Eastern
LAST-MODIFIED:19870101T000000Z
BEGIN:STANDARD
DTSTART:19671029T020000
RRULE:FREQ=YEARLY;BYDAY=-1SU;BYMONTH=10
TZOFFSETFROM:-0400
TZOFFSETTO:-0500
TZNAME:Eastern Standard Time (US &amp; Canada)
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:19870405T020000
RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=4
TZOFFSETFROM:-0500
TZOFFSETTO:-0400
TZNAME:Eastern Daylight Time (US &amp; Canada)
END:DAYLIGHT
END:VTIMEZONE
END:VCALENDAR

If possible, I would like to use TZInfo to get this information.   
 From what I can tell, this data is available in TZInfo, but it is  
not accessible through the API.  Could someone give me some pointers  
on how to get all "active" timezone periods for a given date  
(standard & daylight), and all timezone periods for a given date range?

Thanks,
Brandon
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From phil.ross at gmail.com  Tue Jan  9 09:23:20 2007
From: phil.ross at gmail.com (Philip Ross)
Date: Tue, 9 Jan 2007 14:23:20 +0000
Subject: [TZInfo-users] using TZInfo to generate iCalendar VTIMEZONE
In-Reply-To: <E1670EBA-7379-4166-9247-786937C3A0CD@opensoul.org>
References: <E1670EBA-7379-4166-9247-786937C3A0CD@opensoul.org>
Message-ID: <4ac540410701090623u69b47205h62dfd5848f36ad14@mail.gmail.com>

On 09/01/07, Brandon Keepers <brandon at opensoul.org> wrote:
> I'm working on a CalDAV client for Ruby, and part of the icalendar
> specification requires that you define any timezones that you you
> use.  From what I've gathered, this timezone definition should
> include all the rules for dates in the icalendar file.  So, if I have
> dates spanning from Jan 2006 - June 2007, then I would need to
> include 4 rules, the old standard and daylight times, and the new
> standard and daylight times.

You might be interested in a C program called Vzic
(http://dialspace.dial.pipex.com/prod/dialspace/town/pipexdsl/s/asbm26/vzic/).
This program takes the source data from
http://www.twinsun.com/tz/tz-link.htm (that TZInfo also uses) and
produces a set of iCalendar VTIMEZONE files.

> If possible, I would like to use TZInfo to get this information.
>  From what I can tell, this data is available in TZInfo, but it is
> not accessible through the API.  Could someone give me some pointers
> on how to get all "active" timezone periods for a given date
> (standard & daylight), and all timezone periods for a given date range?

There will only be one timezone period active for any UTC datetime.
There can be zero or more active for a local datetime.  The methods
Timezone#period_for_utc and Timezone#periods_for_local can be used to
obtain TimezonePeriods for a given time. You could then use the
TimezonePeriod#(utc|local)_(start|end) methods to scan forward or back
to find other periods.

When packaging a TZInfo release, the source data is read and converted
(by TZInfo::TZDataParser) into a set of transitions for each zone -
one transition for each time the offset to UTC or zone name changes.
You can access this data, but not through the public interface (so
this may not work in future releases):

module TZInfo
  class DataTimezoneInfo
    attr_reader :transitions
  end
end

tz = TZInfo::Timezone.get('Europe/London')
transitions = tz.send(:info).transitions

transitions will be an array of TimezoneTransitionInfo classes.  See
the source for timezone_transition_info.rb
(http://rubyforge.org/viewvc/trunk/tzinfo/lib/tzinfo/timezone_transition_info.rb?revision=125&root=tzinfo&view=markup)
to see the available methods.

This will only work on 'data' timezones
(tz.kind_of?(TZInfo::DataTimezone)).  Many of the zones are 'linked'
(essentially symbolic links to other zones).  You can find the linked
to timezone by adding a attr_reader to the LinkedTimezone class:

module TZInfo
  class LinkedTimezone
    attr_reader :linked_timezone
  end
end

tz = TZInfo::Timezone.get('GB-Eire').linked_timezone
transitions = tz.send(:info).transitions

I hope this has been of some help to you. Please let me know if you
have any further questions.

-- 
Phil Ross
http://tzinfo.rubyforge.org/ -- DST-aware timezone library for Ruby