Ruby converting UTC to user's time zone - ruby

Question

I have dstring as saved in UTC. While I have user's timezone's offset standard_offset
What I want to show that date in user's timezone after conversion. So this is what I do, but at the end you can see it shows UTC not PST or PDT
[64] pry> dstring
=> "2013-10-31T23:10:50Z"
[65] pry> standard_offset = -8
=> -8
[66] pry> e = Time.parse(dstring) + (standard_offset * 3600)
=> 2013-10-31 15:10:50 UTC
[67] pry> e.strftime("%m/%m/%Y %I:%M %p %Z")
=> "10/10/2013 03:10 PM UTC"
I expect finally to get 10/10/2013 03:10 PM PST How to get that? Note: This is not a rails app.

Answer 1

I added a method in_timezone method in Time class as follows:
class Time
require 'tzinfo'
# tzstring e.g. 'America/Los_Angeles'
def in_timezone tzstring
tz = TZInfo::Timezone.get tzstring
p = tz.period_for_utc self
e = self + p.utc_offset
"#{e.strftime("%m/%d/%Y %I:%M %p")} #{p.zone_identifier}"
end
end
How to use it:
t = Time.parse("2013-11-01T21:19:00Z")
t.in_timezone 'America/Los_Angeles'

Answer 2

Apparently this is an issue with the Ruby standard library.
Sources:
http://rubydoc.info/gems/tzinfo/file/README.md
Note that the local Time returned will have a UTC timezone (local.zone will return "UTC"). This is because the Ruby Time class only supports two timezones: UTC and the current system local timezone.
http://librelist.com/browser//usp.ruby/2011/9/24/unix-time-and-the-ruby-time-class/
Modern kernels do not know nor care about timezones. Conversions from
UTC to the local timezone (and vice versa) are done in user space[2].
Different processes running concurrently on the same machine do not
necessarily share the same local timezone.
Process timezone
The "TZ" environment variable controls the timezone for a given
process, and thus the timezone attached to a Ruby Time object. If
"TZ" is unset, the timezone of the process is implementation-defined.
As far as I can tell, everything related to time zones that is in Rails was built by the Rails core team. Ruby only handles as much time-related functionality as Unix provides and probably expects the user to handle the reset.

Answer 3

require 'tzinfo'
class Time
def in_timezone(tzstring)
tz = TZInfo::Timezone.get(tzstring)
p = tz.period_for_utc(self.utc)
# puts "#{tzstring} -> utc_offset=#{p.utc_offset},utc_total_offset=#{p.utc_total_offset},p.offset=#{p.offset}"
e = self.utc + p.utc_total_offset
"#{e.strftime('%Y-%d-%m %H:%M:%S')} #{p.zone_identifier}"
end
end
[Time.parse("2013-10-20T21:19:00Z"), Time.parse("2013-11-20T21:19:00Z")].each do |t|
puts '=======================================================> ' + t.to_s
puts "\t" + t.in_timezone('GMT')
puts "\t" + "------------------"
puts "\t" + t.in_timezone('Europe/London')
puts "\t" + t.in_timezone('Europe/Prague')
puts "\t" + t.in_timezone('Asia/Jerusalem')
end