Published by Dan on May 12, 2015

Time Zones and datetime_select

Filed under Tips

The Problem

datetime_select and time_zone_select don't work together. If you use both of them and Time.zone doesn't match the selected timezone, the user will unwittingly set the wrong time:

The Problem

The Fix

Use the selected timezone when setting the timestamp:

Time.use_zone(check_in_params[:timezone]) do
  @check_in.attributes = check_in_params
end

The Fix

An Example

  • Users can check-in to places like foursquare
  • Users can backdate check-ins in case they forgot
  • Users view check-in timestamps in the check-in timezone
class CheckIn < ActiveRecord::Base

  # convert the timestamp into the correct timezone
  def timestamp
    super.in_time_zone(timezone) if super
  end

end
class CheckInsController < ApplicationController
  before_action :load_check_in

  def update
    Time.use_zone(check_in_params[:timezone]) do
      @check_in.attributes = check_in_params
    end

    if @check_in.save
      redirect_to action: 'show'
    else
      render action: 'edit'
    end
  end

  private

  def check_in_params
    params.require(:check_in).permit(:timestamp, :timezone)
  end

  def load_check_in
    @check_in = current_user.check_ins.find(params[:id])
  end

end
<!-- edit.html.erb -->
<%= form_for @check_in do |f| %>
  <p><%= f.datetime_select  :timestamp %></p>
  <p><%= f.time_zone_select :timezone %></p>
<% end %>
<!-- show.html.erb -->
<p>Checked in at <%= @check_in.timestamp.strftime("%-I:%M %p %Z") %></p>

Wrap-up

Ideally, datetime_select would support timezone as a piece of the datetime like it does for hours, minutes, and seconds, but it doesn't. The next step is overriding timestamp= but will the timezone attribute be set before or after the timestamp attribute? And what determines the timestamp's internal timezone?

Time.use_zone makes the most sense: the model can't guarantee what the Time.zone value or the timestamp's timezone mean, but the controller can.

Have a better solution? Let me know!

Dan

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