Handle Timezone in Telemedicine apps using Ruby on Rails

Recently I worked on an appointment booking(Telemedicine) feature for one of our healthcare projects. Creating an appointment is all about Date/Time and the challenging part is timezone.
It is so easy to get this done, only if you have the right understanding of how to handle Date and Time along with timezone.

Always save Date/Time in UTC in your database, this way you would be able to convert the DateTime to any timezone you want.
Rails ActiveSupport::TimeZone serves as a wrapper around TZInfo::Timezone instances.
When you get Date, Time and Timezone from the user, set the timezone first, There are two ways to do that in Rails

Option 1
Time.zone = timezone 
Option 2
Time.use_zone(timezone) do
  #Play with the date & time
end

If you want to set the timezone for a particular block go with option 2.

Timezone view helper

Here comes some interesting part of timezones, the daylight savings time.
Rails provide time_zone_select helper to list all the timezones but there is one small thing we have to notice about the Daylight savings.
Let’s take Pacific Time (US & Canada)

In case of using the helper, you will always see Pacific Time (US & Canada)(GMT-08:00) but during the DayLight time it should be Pacific Time (US & Canada)(GMT-07:00)

To fix this we can go with a custom timezone select box with the list of timezones. e.now.formatted_offset will take care of the Daylight offset based on the current time.

timezones = ActiveSupport::TimeZone.all.map{|e| ["#{e.name}(GMT#{e.now.formatted_offset})", e.name]}
<%= select :user, :timezone, timezones, select_attributes, field_attributes %>

Save and Retrieve Date and Time

Now save the date and time in the database by mentioning the timezone.
To parse date and time, instead of Time.parse use Time.zone.parse

Time.use_zone(timezone) do
  appointment.event_datetime = Time.zone.parse("#{date.year}-#{date.month}-#{date.day} #{start_time}")
  appointment.save
end

It’s all set, the date and time are saved in UTC based on the user-selected timezone( along with daylight). Now you can easily retrieve the date and time for any timezone you want using

(appointment.event_datetime).in_time_zone(timezone)
or 
Time.use_zone(timezone) do
  appointment.event_datetime
end
or
Time.zone = timezone
appointment.event_datetime

Few tips while working on timezone

Time.parse('2020-02-12 05:33:39")  -> Time.zone.parse('2020-02-12 05:33:39") 
Time.strptime(string, format) ->  Time.strptime(string, format).in_time_zone
Time.now -> Time.zone.now 
Time.today -> Time.zone.today 

I hope this will help you to understand and implement timezone based feature on your application.