From eschwartz at comverge.com  Tue Oct  4 14:09:25 2011
From: eschwartz at comverge.com (Eric Schwartz)
Date: Tue, 4 Oct 2011 18:09:25 +0000
Subject: [TZInfo-users] DST start/stop times?
Message-ID: <D0FBE0AD-7FE2-4319-AEFB-F1CE0D9DA5BB@comverge.com>

I need to program a device that tracks DST by setting start/stop times, but I don't see a way to get at that information from TZInfo... am I missing something or is there no way other than iterating over every day to figure out when DST starts and ends each year?

-=Eric
-- 
Eric Schwartz
Software Engineer, Comverge, Inc
w: 720.253.0277 m: 630.447.9675
e: eschwartz at comverge.com






From phil.ross at gmail.com  Tue Oct  4 19:13:03 2011
From: phil.ross at gmail.com (Philip Ross)
Date: Wed, 5 Oct 2011 00:13:03 +0100
Subject: [TZInfo-users] DST start/stop times?
In-Reply-To: <D0FBE0AD-7FE2-4319-AEFB-F1CE0D9DA5BB@comverge.com>
References: <D0FBE0AD-7FE2-4319-AEFB-F1CE0D9DA5BB@comverge.com>
Message-ID: <CAMhnMW2Y1zztjQWjJy2R6rD4H6Uz0vc0z_wJ6ObRtGY1WyO0-w@mail.gmail.com>

Hi Eric,

On 4 October 2011 19:09, Eric Schwartz <eschwartz at comverge.com> wrote:
> I need to program a device that tracks DST by setting start/stop times, but I don't see a way to get at that information from TZInfo... am I missing something or is there no way other than iterating over every day to figure out when DST starts and ends each year?

Your best bet would be to use the period_for_utc method. For a given
date and time, this method will return a TimezonePeriod object that
will give you the UTC offset at that time and when the period starts
and ends (i.e. the previous and next daylight savings transitions).

You can find the previous or next TimezonePeriod by either subtracting
a second from the start time or adding a second to the end time and
calling period_for_utc again.

The following example will print all the daylight savings and standard
time periods for New York in the years 2011 to 2020:

zone = TZInfo::Timezone.get('America/New_York')
start_year = 2011
end_year = 2020

period = zone.period_for_utc(Time.utc(start_year, 1, 1))

while period
  offset_hours = period.utc_total_offset / 3600
  offset_mins = period.utc_total_offset % 3600
  offset_sign = period.utc_total_offset >= 0 ? '+' : ''
  offset = "#{offset_sign}#{offset_hours}:#{offset_mins.to_s.rjust(2, '0')}"

  puts "GMT#{offset} #{period.abbreviation} " +
       "from #{period.utc_start} #{period.utc_end ?
                                   "until #{period.utc_end}" : ''}"

  # If this period has an end time (i.e. daylight savings hasn't been
abolished),
  # add a second to the end time and find the next period.
  period = period.utc_end && period.utc_end.year <= end_year ?
              zone.period_for_utc(period.utc_end + Rational(1,86400)) :
              nil
end


Kind regards,

Phil

From eschwartz at comverge.com  Wed Oct  5 11:11:28 2011
From: eschwartz at comverge.com (Eric Schwartz)
Date: Wed, 5 Oct 2011 15:11:28 +0000
Subject: [TZInfo-users] DST start/stop times?
In-Reply-To: <CAMhnMW2Y1zztjQWjJy2R6rD4H6Uz0vc0z_wJ6ObRtGY1WyO0-w@mail.gmail.com>
References: <D0FBE0AD-7FE2-4319-AEFB-F1CE0D9DA5BB@comverge.com>
	<CAMhnMW2Y1zztjQWjJy2R6rD4H6Uz0vc0z_wJ6ObRtGY1WyO0-w@mail.gmail.com>
Message-ID: <FA352B4C-9BAA-4311-9246-1C156BA60CEB@comverge.com>

Thanks; that was exactly what I needed. In my Copious Free Time(tm), I would be inclined to wrap that up into a method along these lines

def TZInfo::Timezone#period_for_dst(year=Time.now.year)
  # stuff
end

if that's a patch you might be interested in. No worries if not; I can understand wanting to keep such a widely-used library lean.

-=Eric
-- 
Eric Schwartz
Software Engineer, Comverge, Inc
w: 720.253.0277 m: 630.447.9675
e: eschwartz at comverge.com





On Oct 4, 2011, at 5:13 PM, Philip Ross wrote:

> Hi Eric,
> 
> On 4 October 2011 19:09, Eric Schwartz <eschwartz at comverge.com> wrote:
>> I need to program a device that tracks DST by setting start/stop times, but I don't see a way to get at that information from TZInfo... am I missing something or is there no way other than iterating over every day to figure out when DST starts and ends each year?
> 
> Your best bet would be to use the period_for_utc method. For a given
> date and time, this method will return a TimezonePeriod object that
> will give you the UTC offset at that time and when the period starts
> and ends (i.e. the previous and next daylight savings transitions).
> 
> You can find the previous or next TimezonePeriod by either subtracting
> a second from the start time or adding a second to the end time and
> calling period_for_utc again.
> 
> The following example will print all the daylight savings and standard
> time periods for New York in the years 2011 to 2020:
> 
> zone = TZInfo::Timezone.get('America/New_York')
> start_year = 2011
> end_year = 2020
> 
> period = zone.period_for_utc(Time.utc(start_year, 1, 1))
> 
> while period
>  offset_hours = period.utc_total_offset / 3600
>  offset_mins = period.utc_total_offset % 3600
>  offset_sign = period.utc_total_offset >= 0 ? '+' : ''
>  offset = "#{offset_sign}#{offset_hours}:#{offset_mins.to_s.rjust(2, '0')}"
> 
>  puts "GMT#{offset} #{period.abbreviation} " +
>       "from #{period.utc_start} #{period.utc_end ?
>                                   "until #{period.utc_end}" : ''}"
> 
>  # If this period has an end time (i.e. daylight savings hasn't been
> abolished),
>  # add a second to the end time and find the next period.
>  period = period.utc_end && period.utc_end.year <= end_year ?
>              zone.period_for_utc(period.utc_end + Rational(1,86400)) :
>              nil
> end
> 
> 
> Kind regards,
> 
> Phil
> _______________________________________________
> TZInfo-users mailing list
> TZInfo-users at rubyforge.org
> http://rubyforge.org/mailman/listinfo/tzinfo-users


From phil.ross at gmail.com  Sun Oct  9 11:18:15 2011
From: phil.ross at gmail.com (Philip Ross)
Date: Sun, 9 Oct 2011 16:18:15 +0100
Subject: [TZInfo-users] DST start/stop times?
In-Reply-To: <FA352B4C-9BAA-4311-9246-1C156BA60CEB@comverge.com>
References: <D0FBE0AD-7FE2-4319-AEFB-F1CE0D9DA5BB@comverge.com>
	<CAMhnMW2Y1zztjQWjJy2R6rD4H6Uz0vc0z_wJ6ObRtGY1WyO0-w@mail.gmail.com>
	<FA352B4C-9BAA-4311-9246-1C156BA60CEB@comverge.com>
Message-ID: <CAMhnMW0EK_Ycq8jN=AvhWBM8AKTcjD9pUjC2ixEUz5OG=TSnFA@mail.gmail.com>

Hi Eric,

On 5 October 2011 16:11, Eric Schwartz <eschwartz at comverge.com> wrote:
> Thanks; that was exactly what I needed. In my Copious Free Time(tm), I would be inclined to wrap that up into a method along these lines
>
> def TZInfo::Timezone#period_for_dst(year=Time.now.year)
> ?# stuff
> end
>
> if that's a patch you might be interested in. No worries if not; I can understand wanting to keep such a widely-used library lean.

Yes, it seems like something along these lines would be useful, so I'd
be interested in a patch if you can find the time.

I think methods that returned an array of TimezonePeriod objects
between two dates (one method for UTC and another for local time)
would be more generally useful than just returning the periods in a
year:

def TZInfo::Timezone#periods_between_utc(utc_from, utc_to)
  # stuff
end

def TZInfo::Timezone#periods_between_local(local_from, local_to, dst =
Timezone.default_dst)
  # stuff
end

The from and to parameters will have to accept Time, DateTime and
Numeric parameters (see TZInfo::TimeOrDateTime). The local time
version of the method will have to resolve ambiguous local times (see
TZInfo::Timezone#period_for_local).

Kind regards,

Phil