Time Zone for rails

Time zones

There are trhee diffrent time zones in rails

System - The server time zone
Application - Rails application use own time zone
Database - Default is UTC, but do not change it

Cautions

config.time_zone should be set when an app dose not support multiple time zones
Do not use Date/Time API using system time zone
Date could be incorrect by being converted from datetime or time to date
Use date type instead of datetime type for handing only date if an app dose not support multiple time zones
datetime and in_time_zone should be used in multiple time zones
Do not use DateTime.new for saving and querying because time zone is not applied unless an app use UTC time zone
Use find_zone to make specific time zone time
Do not use in_time_zone with new for DateTime or Time to make specific time zone time

Difference among date/time API

Application vs System time

Application time zone - Default is UTC unless Time.zone="Alaska" or config.time_zone = "Pacific/Auckland"
System - This depends on how Server OS time is set up

Do’s and Don'ts with time zones

Do not use date/time API using the system time zone

# Do not use below, because it uses the system time zone 
Time.now
DateTime.now
Date.today
Date.today.to_time
Time.parse("2015-07-04 17:05:37")
Time.strptime(string, "%Y-%m-%dT%H:%M:%S%z")
Time.strptime("2020-02-19 00:00","%Y-%m-%d %H:%M").in_time_zone

# Use below
Time.zone.now
Time.zone.today
Time.current
2.hours.ago
Date.current
1.day.from_now
Time.zone.parse("2015-07-04 17:05:37")

Also, Do not use Time.zone in your application code

# bad smell
Time.zone = "Alaska"

# use below
# After executing the block, the original time zone is set back 
Time.use_zone("Alaska", &block)

Convert time zone

When datetime or time data is saved or read from database, the data is converted accroing to timezone

Save & Query - The datetime is changed to UTC
Read - The datetime is changed to application timezone

Save data

# published is datetime
# and current time is Tue, 25 Feb 2020 12:26:36 in New Zealand

# time zone is set with "Pacific/Auckland"
Product.create!(published: Date.new(2020,02,24))
# The published is 2020-02-23 11:00:00, which is UTC

Product.create!(published: Time.new(2020,02,24))
# The published is 2020-02-23 11:00:00

Product.create!(published: Date.today)
# The published is 2020-02-24 11:00:00

Product.create!(published: Date.current)
# The published is 2020-02-24 11:00:00


# time zone is not set, so "UTC"
Product.create!(published: Date.new(2020,02,24))
# The published is 2020-02-24 00:00:00
Product.create!(published: Date.new(2020,02,24).in_time_zone("Pacific/Auckland"))
# The published is 2020-02-23 11:00:00

Product.create!(published: Time.new(2020,02,24))
# The published is 2020-02-23 11:00:00

Product.create!(published: Date.today)
# The published is 2020-02-25 00:00:00

Product.create!(published: Date.current)
# The published is 2020-02-24 00:00:00

All other datetime columns like created_at and updated_at are also UTC
Rails record is converted to UTC using to_s(:db)

Date.new(2020,02,24).to_s(:db)
=> "2020-02-24"

# in order to check saving to datetime type column, should add in_time_zone
Date.new(2020,02,24).in_time_zone.to_s(:db)
=> "2020-02-23 11:00:00"

Time.new(2020,02,24).in_time_zone.to_s(:db)
=> "2020-02-23 11:00:00"

Read data

# time zone is set with "Pacific/Auckland"
# The published is 2020-02-23 11:00:00
Product.last.published
 => Mon, 24 Feb 2020 00:00:00 NZDT +13:00
 
# The published is "2020-02-24 00:00:00
Product.last.published
=> Mon, 24 Feb 2020 13:00:00 NZDT +13:00
 
 
# time zone is not set, so "UTC"
# The published is 2020-02-23 11:00:00
Product.last.published
 => Sun, 23 Feb 2020 11:00:00 UTC +00:00
 
# The published is "2020-02-24 00:00:00 
Product.last.published
=> Mon, 24 Feb 2020 00:00:00 UTC +00:00

Incorrect date

When saving and displaying date, the date could be incorrect if timezone is just default UTC

Read

# timezone is UTC
# created_at is 2020-02-23 23:26:36
Product.last.created_at
 => Sun, 23 Feb 2020 23:26:36 UTC +00:00

Product.last.created_at.to_date
=> Sun, 23 Feb 2020 # Should be Mon, 24 Feb 2020


# time zone is set with "Pacific/Auckland"
Product.last.created_at
=> Mon, 24 Feb 2020 12:26:36 NZDT +13:00

Product.last.created_at.to_date
Product.last.created_at.in_time_zone("Pacific/Auckland")
 => Mon, 24 Feb 2020

Save & Query

Also date could be incorrect when time information is cutted

# timezone is just default timezone, so UTC
# current time is Mon, 24 Feb 2020 12:26:36 in New Zealand
# published_date is date type
Product.create!(published_date:Time.current)
# published_date is 2020-02-23 because Sun, 23 Feb 2020 23:26:36 UTC +00:00

Product.create!(published_date:Date.current)
# published_date is 2020-02-23 because Date.current is Sun, 23 Feb 2020 23:26:36 UTC +00:00 in UTC

Product.create!(published: Date.current.in_time_zone("Pacific/Auckland"))
# The published is 2020-02-22 11:00:00 because Date.current is 23 
Product.where(published: Date.current.in_time_zone("Pacific/Auckland"))

How to display date time

An application does not support multi-time zone, we can disply datetime correctly by setting config.time_zone
However, if an application support it, displaying datetime could be a bit complicated.

Change datetime to Browser local time
Save timezone information into database and use it
Sending zone information

Change datetime to broswer time

There are javascript functions(Date(string)) and libraries
gem local_time could be usefull

Save timezone information

when the request completes, the original time zone is set back.

create_table :users do |t|
  t.string :time_zone, default: "UTC"
end

# input user time zone, simpleform can support :time_zone 
<%= f.input :time_zone %> 

class ApplicationController < ActionController::Base
  around_action :set_current_user_timezone, if: :current_user
  
   def set_current_user_timezone(&block)
     Time.use_zone(current_user.time_zone, &block)
   end
end

# Displaying datetime
<%= time.in_time_zone(current_user.time_zone) %>

Sending zone information

When request for displaying or query,

Sending time zone information(jstz )
Sending zone offset(getTimezoneOffset())

These information can be saved in session or with every request.

How to save and query date

Single time zone

config.time_zone should be set.

date

When saving and query date, date type could be useful if an application do not care less about multiple time zones

# published_date is date type
Product.create!(published_date: Date.current)
Product.create!(published_date: Date.new(2020,02,24))

Product.where(published_date:Date.current)
Product.where(published_date: Date.new(2020,02,24)..Date.new(2020,02,25))
Product.where("published_date <= :date", date: Date.new(2011,6,1)).order(published_date: :asc).last

datetime

When using datetime type for date, insert and query data could be a little complicated than date
Shoud use in_time_zone or beginning_of_day/ end_of_day becasue it includes time information

# published is datedate type
# 2020-02-24 11:00:00
# 2020-02-23 11:00:00

Product.create!(published: Date.current)

Date.current
=> Tue, 25 Feb 2020
Date.current.in_time_zone 
=> Tue, 25 Feb 2020 00:00:00 NZDT +13:00
Date.current.beginning_of_day
=> Tue, 25 Feb 2020 00:00:00 NZDT +13:00

# Today
Product.where(published: Date.current.in_time_zone) 
Product.where(published: Date.current.beginning_of_day .. Date.current.end_of_day)
Product.where(published: Date.current.all_day)

1.day.ago
 => Mon, 24 Feb 2020 16:49:25 NZDT +13:00
Product.where(published: 1.day.ago.beginning_of_day .. Date.current.end_of_day)

Product.where(published: Date.new(2020,2,24).in_time_zone ..  Date.new(2020,2,25).in_time_zone)

Multiple time zones

If an application supports several time zones, datetime and in_time_zone should be used
To avoid trublemsome, save datetime or time format

# published is datetime
Product.create!(published: DateTime.current)
# published is saved as "2020-02-25 21:10:36.033901"

Product.where(published: DateTime.current.in_time_zone("Alaska").all_day)

# To use find_zone when saving or query specific date and time
Time.find_zone("Alaska").local(2020,02,19).to_s(:db)
 => "2020-02-19 09:00:00"

in_time_zone vs find_zone

in_time_zone is used for changing datetime which was made to time zone date time
To make time zone datetime , use find_zone

DateTime - Has date and time information, utc time
Date - Only has information about date, application time zone
Time - It is system time

Date.new(2020,02,19)
=> Wed, 19 Feb 2020 

Date.new(2020,02,19).in_time_zone("Alaska")
=> Wed, 19 Feb 2020 00:00:00 AKST -09:00

Date.new(2020,02,19).in_time_zone("Alaska").to_s(:db)
=> "2020-02-19 09:00:00"
 
DateTime.new(2020,02,19)
=> Wed, 19 Feb 2020 00:00:00 +0000

DateTime.new(2020,02,19).in_time_zone("Alaska")
=> Tue, 18 Feb 2020 15:00:00 AKST -09:00 

DateTime.new(2020,02,19).in_time_zone("Alaska").to_s(:db)
 => "2020-02-19 00:00:00"

Time.new(2020,2,19) #system time zone
=> 2020-02-19 00:00:00 +1300

# Time.new creates system time, which is 2020-02-19 00:00:00 +1300
Time.new(2020,02,19).in_time_zone("Alaska")
=> Tue, 18 Feb 2020 02:00:00 AKST -09:00

Time.new(2020,02,19).in_time_zone("Alaska").to_s(:db) 
 => "2020-02-18 11:00:00" #system is "Pacific/Auckland"
 
Time.new(2020,02,19).in_time_zone("UTC").to_s(:db) 
 => "2020-02-18 11:00:00 

Time.find_zone("Alaska").local(2020,02,19).to_s(:db)
 => "2020-02-19 09:00:00"

JamesYang76/Time Zone.md