ActiveSupport::Duration incorrectly calculates interval? - ruby-on-rails

Question

I have a very strange feeling that I am getting incorrect duration calculated by ActiveSupport::Duration. Here is the essence of the code I have
require 'time'
require 'active_support/duration'
require 'active_support/gem_version'
a = Time.parse('2044-11-18 01:00:00 -0600')
b = Time.parse('2045-03-05 04:00:00 -0600')
ActiveSupport::Duration.build(b - a).inspect
ActiveSupport.gem_version
And here is what I get
[30] pry(main)> require 'time'
=> false
[31] pry(main)> require 'active_support/duration'
=> false
[32] pry(main)> require 'active_support/gem_version'
=> false
[33] pry(main)> a = Time.parse('2044-11-18 01:00:00 -0600')
=> 2044-11-18 01:00:00 -0600
[34] pry(main)> b = Time.parse('2045-03-05 04:00:00 -0600')
=> 2045-03-05 04:00:00 -0600
[35] pry(main)> ActiveSupport::Duration.build(b - a).inspect
=> "3 months, 2 weeks, 1 day, 19 hours, 32 minutes, and 42.0 seconds"
[36] pry(main)> ActiveSupport.gem_version
=> Gem::Version.new("6.0.1")
I cross-checked the result with PostgreSQL
select justify_interval('2045-03-05 04:00:00 -0600'::timestamp - '2044-11-18 01:00:00 -0600'::timestamp)
and got 3 mons 17 days 03:00:00 (or 107 days and 3 hours). Also there is a web site that gives result consistent with PostgreSQL (although web page says 107 days are 3 months and 15 days).
Am I missing something? Where minutes and seconds are coming from? Is there a better interval calculator for Ruby/Rails?
Update
distance_of_time_in_words returns 4 months!
Update 2
I ended up with slightly modified Wizard's solution wrapped up to produce text
def nice_duration(seconds)
parts = duration_in_whms(seconds)
out = []
I18n.with_options(scope: 'datetime.distance_in_words') do |locale|
out.push locale.t(:x_days, count: parts[:days]) if parts.key?(:days)
out.push locale.t(:x_hours, count: parts[:hours]) if parts.key?(:hours)
out.push locale.t(:x_minutes, count: parts[:minutes]) if parts.key?(:minutes)
end
out.join ' '
end
private
def duration_in_whms(seconds)
parts_and_seconds_in_part = {:days => 86400, :hours => 3600, :minutes => 60}
result = {}
remainder = seconds
parts_and_seconds_in_part.each do |k, v|
out = (remainder / v).to_i
result[k] = out if out.positive?
remainder -= out * v
end
result.merge(seconds: remainder)
end
Apparently localization from Action View does not have hours without about. So I also had to add corresponding translation into my locales
en:
datetime:
distance_in_words:
x_hours:
one: "1 hour"
other: "%{count} hours"

Answer 1

ActiveSupport::Duration calculates its value using the following constants and algorithm (I have added the explanation on what it's doing below but here is a link to the source). As you can see below, the SECONDS_PER_YEAR constant is the average number of seconds in the gregorian calendar (which is then used to define SECONDS_PER_MONTH). It is because of this, "average definition" of SECONDS_PER_YEAR and SECONDS_PER_MONTH that you are getting the unexpected hours, minutes and seconds. It is defined as an average because a month and year is not a standard fixed amount of time.
SECONDS_PER_MINUTE = 60
SECONDS_PER_HOUR = 3600
SECONDS_PER_DAY = 86400
SECONDS_PER_WEEK = 604800
SECONDS_PER_MONTH = 2629746 # This is 1/12 of a Gregorian year
SECONDS_PER_YEAR = 31556952 # The length of a Gregorian year = 365.2425 days
# You pass ActiveSupport::Duration the number of seconds (b-a) = 9255600.0 seconds
remainder_seconds = 9255600.0
# Figure out how many years fit into the seconds using integer division.
years = (remainder_seconds/SECONDS_PER_YEAR).to_i # => 0
# Subtract the amount of years from the remaining_seconds
remainder_seconds -= years * SECONDS_PER_YEAR # => 9255600.0
months = (remainder_seconds/SECONDS_PER_MONTH).to_i # => 3
remainder_seconds -= months * SECONDS_PER_MONTH # => 1366362.0
weeks = (remainder_seconds/SECONDS_PER_WEEK).to_i # => 2
remainder_seconds -= weeks * SECONDS_PER_WEEK # => 156762.0
days = (remainder_seconds/SECONDS_PER_DAY).to_i # => 1
remainder_seconds -= days * SECONDS_PER_DAY # => 70362.0
hours = (remainder_seconds/SECONDS_PER_HOUR).to_i # => 19
remainder_seconds -= hours * SECONDS_PER_HOUR # => 1962.0
minutes = (remainder_seconds/SECONDS_PER_MINUTE).to_i # => 32
remainder_seconds -= minutes * SECONDS_PER_MINUTE # => 42
seconds = remainder_seconds # => 42
puts "#{years} years, #{months} months, #{weeks} weeks, #{days} days, #{hours} hours, #{minutes} minutes, #{seconds} seconds"
# 0 years, 3 months, 2 weeks, 1 days, 19 hours, 32 minutes, 42.0 seconds
To avoid the issue you are having, I would suggest to just represent the time in week, days, hours, minutes and seconds (basically anything excluding month & year).
The number of seconds in a month is complicated if you don't use an average since you will need to account for 28, 29, 30 and 31 days for each separate month. Similarly, for the year, you will need to account for leap/non-leap if you don't use the average.
I am not sure of any gems around which do this for you, however I can provide a function which can help you calculate the duration in days, hours, minutes and seconds below.
def duration_in_whms(seconds)
parts_and_seconds_in_part = {:weeks => 604800, :days => 86400, :hours => 3600, :minutes => 60}
result = {}
remainder = seconds
parts_and_seconds_in_part.each do |k, v|
result[k] = (remainder/v).to_i
remainder -= result[k]*v
end
result.merge(seconds: remainder)
end
duration_in_whms(9255600) => # {:weeks=>15, :days=>2, :hours=>3, :minutes=>0, :seconds=>0.0}