Imagine you have an app that sends an email to users every morning at 8am. Maybe its a to-do app and you want to send a list of things to get done every morning. The problem is that your users live all over the world. You have the time zone for each user, but how do you know when it is 8:00 in each one?
I was working on this problem for GroupBuzz and found that it was a little trickier than I expected. This is what I did and how you can do it too.
First, there are a few things you should know about time zones. They change. New zones are added and existing zones adjusted. Some of them have offsets that change during daylight saving and when that offset begins and ends can change from year to year. On top of that, some offsets are not whole hours. For example, Australia/Eucla is UTC+08:45 and Asia/Kolkata is UTC+05:30.
Rails uses the TZinfo gem to get information about time zones as well as translating times between zones. Our problem is that we want to go the other direction. At any given time on our server, how do we know if it is 8:00am local time for any of our users?
That might sound inefficient, but it isnt that bad and you only need to check the zones that you have for your users.
You gain simplicity. It isnt the job of your app to keep track of time zone data so dont do it. Let TZInfo
do that for you.
To illustrate the approach take a look at the script below. It starts at midnight and checks all zones for each 15 minute increment for a whole day. Along the way it prints the time in UTC and the zones where it is locally 8:00.
The output will look something like this:
00:00 UTC => 08:00 in Antarctica/Casey 00:00 UTC => 08:00 in Asia/Brunei 00:00 UTC => 08:00 in Asia/Chita ... 15:00 UTC => 08:00 in US/Pacific ... 22:30 UTC => 08:00 in Australia/South ... 23:15 UTC => 08:00 in Australia/Eucla
I included a couple interesting zones in Australia to show that not all offsets are in full hour increments. I live in California, which, in May 2015, has an offset of UTC-7. Therefore 8:00 in California should occur at 15:00 UTC. Occording to our script, that is correct. Yay! Unfortunately, after November 8, the offset becomes UTC-8 and 8:00 local time in California occurs at 16:00 UTC. This is the key. For any time zone we don't know when daylight saving begins and ends or is observed at all. Of course we could know. We could look it up, but that's not the business we want to be in. It is the business of TZInfo and the maintainers do an awesome job.
At any moment in time, we can find time zones that have a specific local time. So far we've been using 8:00 as an example, but it could be any hour of the day.
Let's take it further and see if we can make it more useful by applying it to a possible use case in a rails app. Back to the to-do app. Assuming we have an ActiveRecord based User model with at least two columns: email and time_zone_name . Every 15 minutes we'll check the unique time zones in our system to see if it's 8:00 local time. For all the zones that it is, we'll send our "to-do today" email.
Let's start by putting the logic to find zones with a specified local time in a class called TimeZoneSearch with a descriptive method "zones_with_local_hour". Using it to do the 8:00 search would look like this:
TimeZoneSearch.new(TZInfo::Timezone.all_identifiers).zones_with_local_hour(8)
This is the full implementation:
We're giving it the zones to search because determining which zones to search will come from querying the users table. Below is the User class with a class method to lookup all the zones being used and a scope to find users by time zone name.
We can combine these to send out an email at 8:00. I like to define a class for this so that it can easily be called from the console, a rails runner, or a rake task.
This is easily wrapped in a rake task or called with a rails runner direclty from the command line.
bundle exec rails -r "TodoSummaryJob.send_daily_summary"
By running this every 15 minutes we can be confident that our users will be getting their to-do lists at the right time year round. I'm leaving it up to you to create UserMailer and Todo with an association to User. =)
How are you using time zones in your app?
I'm always looking for new topics to write about. Stuck on a problem or working on something interesting? You can reach me on Twitter @bradpauly or send me an email.