A memo of how ri_cal should be used to deal with timezones. It converts http://ambulante.com.mx's schedule to ICS format too.

ambulante_ics.rb

#!/usr/bin/env ruby
# encoding: utf-8
#
# What:
#
# A memo of how ri_cal should be used to deal with timezones.  It converts
# ambulante.com.mx's schedule to ICS format too.
#
# Deps:
#
#   - nokogiri
#   - tzinfo
#   - ri_cal
#   - chronic
#
# Usage:
#
# Grab the schedule's URL, then invoke with:
#   curl $URL | ruby ambulante_ics.rb > Ambulante.ics

require 'nokogiri'
require 'tzinfo'
require 'ri_cal'
require 'chronic'

TZ = 'America/Mexico_City' # Times on the schedule are on this time zone
ENCODING = 'iso-8859-1'    # website data isn't UTF8...

class Nokogiri::XML::Element
  def join_children_text(join_string)
    children.map(&:text).find_all { |txt| txt != '' }.join(join_string)
  end
end

# Parse HTML DOM, and return a list of events.
def extract_events(doc)
  doc.css('.lista_funciones tr')[1..-1].map do |event|
    html_date, html_details, _, html_movie, html_place = event.children / 'td'
    # Parse date
    date = Chronic.parse html_date.text.sub(/\|\s*([\d:]+) HRS/, '\1')
    # Parse pricing details, and additional informations
    details = html_details.join_children_text(', ')
    # Parse movie details
    movie = html_movie.join_children_text(' (') + ')'
    # Parse duration (default to 1 hour if not found)
    if movie_duration = movie.match(/(\d+) min/)
      duration_in_seconds = movie_duration[1].to_i * 60
    else
      duration_in_seconds = 60 * 60
    end
    # Parse location details
    location = html_place.join_children_text(', ')

    {
      summary:     movie.encode(ENCODING),
      description: details.encode(ENCODING),
      location:    location.encode(ENCODING),
      date_start:  date,
      date_end:    date + duration_in_seconds,
    }
  end
end

# Generate ICS calendar from a list of events
def generate_ics(events)
  RiCal.Calendar do |cal|
    cal.default_tzid = TZ
    events.each do |event|
      cal.event do
        summary     event[:summary]
        description event[:description]
        location    event[:location]
        dtstart     event[:date_start]
        dtend       event[:date_end]
      end
    end
  end
end

events = extract_events Nokogiri::HTML(ARGF)
print generate_ics(events).to_s