Timezone_Rails

event.js

//eonasdan-bootstrap-datetimepicker

jQuery('#datetimepickerStart').datetimepicker({
  sideBySide: true
});
jQuery('#datetimepickerStop').datetimepicker({
  sideBySide: true,
  useCurrent: false //Important! See issue #1075
});
jQuery('#datetimepickerStart').on('dp.change', function (e) {
  jQuery('#datetimepickerStop').data('DateTimePicker').minDate(e.date);
});
jQuery('#datetimepickerStop').on('dp.change', function (e) {
  jQuery('#datetimepickerStart').data('DateTimePicker').maxDate(e.date);
});

// Timezone
var zoneOffset = moment().format('Z')
jQuery('.detectedTimezone').html(jstz.determine().name())

if(jQuery("#eventTimezone select").val() === ""){
  jQuery("#eventTimezone select").val(zoneOffset);
}

jQuery('.detectedOffset').html(zoneOffset);

event.rb

class Event < ActiveRecord::Base
  PARSE_DATE_FORMAT = '%m/%d/%Y %l:%M %p %:z'
  DISPLAY_DATE_FORMAT = '%m/%d/%Y %l:%M %p'

  TIMEZONES = [
    HAWAII   = { hawaii:   '-10:00'},
    ALASKA   = { alaska:   '-09:00'},
    PACIFIC  = { pacific:  '-08:00'},
    MOUNTAIN = { mountain: '-07:00'},
    CENTRAL  = { central:  '-06:00'},
    EASTERN  = { eastern:  '-05:00'},
    ATLANTIC = { atlantic: '-04:00'}
  ].reduce( {}, :merge )

  scope :future,  ->{ where("event_start >= :date", date: Time.now) }
  scope :current, ->{ where("event_start <= ? AND event_end >= ?", Time.now, Time.now) }
  scope :past,    ->{ where("event_end < :date",  date: Time.now) }

  def self.display_timezones
    TIMEZONES.map{ |zone, offset| ["#{zone.capitalize} \t #{offset}", offset] }
  end

  def format_start
    return "" unless event_start
    apply_time_zone(event_start).strftime(Event::DISPLAY_DATE_FORMAT)
  end

  def format_end
    return "" unless event_end
    apply_time_zone(event_end).strftime(Event::DISPLAY_DATE_FORMAT)
  end

  def start_date_ratio_to_end_of_year
    days = days_to_event
    days_in_year = ((DateTime.now.end_of_year - DateTime.now.beginning_of_year).to_i + 1).to_f
    ((days/days_in_year)*100).ceil
  end

  def days_to_event
    event_beginning_on = event_start.to_date
    current_time = Date.today
    days = (event_beginning_on - current_time).to_i
    return 0 if days < 0
    days
  end

private

  def apply_time_zone(datetime)
    tz_offset = time_zone.split(':')[0].to_i
    tz = ActiveSupport::TimeZone[tz_offset]
    datetime.in_time_zone(tz)
  end
end

event_controller.rb

class EventsController < ApplicationController
  before_action :event_from_params, only: [:show, :edit, :update]

  def index
    @events = Event.all
  end

  def show
    @event = Event.find(params[:id])
  end

  def new
    @event = Event.new
  end

  def create
    @event = Event.new
    @event.assign_attributes transform_param_dates

    if @event.valid?
      @event.save
      redirect_to edit_event_path(@event)
    else
      render 'edit'
    end
  end

  def edit; end

  def update
    @event.assign_attributes transform_param_dates

    if @event.valid?
      @event.save
      redirect_to edit_event_path(@event)
    else
      render 'edit'
    end
  end

  def future
    @events = Event.future
    render json: {events: @events}
  end

  def past
    @events = Event.past
    render json: {events: @events}
  end

private

  def parse_event_date_params
    date_keys = ['event_start', 'event_end']
    time_zone = event_params['time_zone']
    event_params
      .select{ |key| date_keys.include?(key) }
      .map{ |key, value| {key => interpret_date(value, time_zone)} }
      .reduce(:merge)
  end

  def interpret_date(str, tz)
    date_time = DateTime.strptime("#{str} #{tz}", Event::PARSE_DATE_FORMAT)
    date_time.to_time.utc
  end

  def event_from_params
    @event = Event.find(params['id'])
  end

  def transform_param_dates
    event_params.merge parse_event_date_params
  end

  def event_params
    params.require(:event).permit(:client_id, :event_type, :event_start, :event_end, :time_zone)
  end
end

event_form.haml

= form_for @event do |f|
  - if @event.errors.any?
    #error_explanation
      %h2= "#{pluralize(@event.errors.count, 'error')} prohibited this contact from being saved:"
      %ul
        - @event.errors.full_messages.each do |msg|
          %li= msg
  .form-group
    .col-md-10.field
      = f.label :event_type, 'Type'
      = f.select(:event_type,  Event::TYPES.collect { |p| p }, {prompt: 'Select One'})

  .form-group
    .col-md-10
      %label Event Dates:
    .col-md-5
      %label Begins:
      #datetimepickerStart.input-group.date
        = f.text_field :event_start, {class: 'form-control', value: @event.format_start}
        .input-group-addon
          %span.glyphicon.glyphicon-calendar
    .col-md-5
      %label Ends:
      #datetimepickerStop.input-group.date
        = f.text_field :event_end, {class: 'form-control', value: @event.format_end}
        .input-group-addon
          %span.glyphicon.glyphicon-calendar

    .col-md-10
      %label Timezone (UTC Offset):
      #eventTimezone.input-group.timezone
        = f.select(:time_zone,  Event.display_timezones, {prompt: 'Select One'})
      %span Detected:
      %span.detectedTimezone
      %span.detectedOffset

  .form-group
    .col-md-10
      %label.col-sm-2.control-label Client
    .col-sm-10
      = f.collection_select :client_id, Client.order(:name),:id,:name, {prompt: '- Select One -'}

  .form-group
    .col-md-10.actions
      = f.submit 'Save', {:class=>"btn btn-default"}
      = link_to "Cancel", :back, {:class=>"btn btn-default"}

The JS also requires moment with locales to handle some of the timezone detection