I read the above answer and it is clear in its delineation of Time, DateTime and Date in Ruby.
Time is packaged with Ruby. It is measured as seconds since January 1, 1970 UTC and is time-zone agnostic. More specifically, the Time class stores integer numbers, which presents the seconds intervals since the Epoch. We can think of this as Unix Time. It has some limitations. I read somewhere if stored as a 64-bit signed integer, it can represent dates between 1823-11-12 to 2116-02-20, but on my system it can represent dates outside this range. If you do not specify the timezone to use in the enviroment variable ENV['TZ'], then it will default to your system time found in /etc/localtime on Unix-like systems. When to use Time? It is useful for measuring time elapse or interpolating a timestamp into a string value.
Rails actually extends the Time class. It accomplishes this through ActiveSupport::TimeWithZone. It provides support for configurable time zones. Note Rails will always convert time zone to UTC before it writes to or reads from the database, no matter what time zone you set in the configuration file. In other words, it is the default behaviour of Rails that all your time will get saved into database in UTC format.
# Get current time using the time zone of current local system or ENV['TZ'] if the latter is set.
Time.now
# Get current time using the time zone of UTC
Time.now.utc
# Get the unix timestamp of current time => 1524855779
Time.now.to_i
# Convert from unix timestamp back to time form
Time.at(1524855779)
# USE Rails implementation of Time! Notice we say Time.current rather than Time.now. This will allow you to use the timezone defined in Rails configuration and get access to all the timezone goodies provided by ActiveSupport::TimeWithZone.
Time.current
TimeWithZone provides a lot of very useful helper methods:
# Get the time of n day, week, month, year ago
1.day.ago
1.week.ago
3.months.ago
1.year.ago
# Get the beginning of or end of the day, week, month ...
Time.now.beginning_of_day
30.days.ago.end_of_day
1.week.ago.end_of_month
# Convert time to unix timestamp
1.week.ago.beginning_of_day.to_i
# Convert time instance to date instance
1.month.ago.to_date
For most cases, the Time with the time zone class from Rails’ ActiveSupport is sufficient. But sometimes you just need a date.
Just as with the Time class, Ruby is packaged with the Date class. Simply require the time library:
require "time"
Time.parse("Dec 8 2015 10:19")
#=> 2015-12-08 10:19:00 -0200
Date.parse("Dec 8 2015")
#=> #<Date: 2015-12-08>
Time.new(2015, 12, 8, 10, 19)
#=> 2015-12-08 10:19:00 -0200
Date.new(2015, 12, 8)
Since Date is part of Ruby, it by default uses the timezone defined in /etc/localtime on Unix-like systems, unless you modify the TZ environmental variable. Just as with the Time class, Rails extends the Date class. Use Date.current instead of Date.today to take advantage of ActiveSupport::TimeWithZone and use Rails-based timezone configurations.
Now there is one more class available with regards to dates and times. DateTime is a subclass of Date and can easily handles date, hour, minute, second and offset. It is both available in Ruby (via require 'time') and in Rails (via require 'date'). Rails extends it with TimeZone capabilities just like with the Time class.
require 'date'
DateTime.new(2001,2,3,4,5,6)
I personally do not see a need for using DateTime in your applications, for you can use Time itself to represent dates and times, and you can use Date to represent dates.
The second part of the question was regarding strptime and strftime. Time, Date and DateTime all have the strptime and strftime methods. strptime parses the given string representation and creates an object. Here is an example:
> result = Time.strptime "04/27/2018", "%m/%d/%Y"
=> 2018-04-27 00:00:00 -0400
> result.class
=> Time
This is useful if you have an application and a user submits a form and you are given a date and/or represented as a string. You will want to parse it into a Time or Date before you save it to the database.
strftime formats a date or time. So you call it on a Date or Time object:
> Date.current.strftime("%Y-%m-%d")
=> "2018-04-27"
And you can use them together to first parse user input and then format it in a certain way, perhaps to output into a csv file:
value = Date.strptime(val, '%m/%d/%Y').strftime('%Y-%m-%d')