One month ago, Ben and I investigated on 1.day not being an Object (that's an interesting post by Ben, I suggest you read it if you want to know what's happening under the hood).

Well I've got news for you, things only get weirder!

Discovering the problem

Let's use Timecop to freeze time first, so that we know where we're at (or should I say when?).

irb(main):001:0> Timecop.freeze '2015/02/19'
=> 2015-02-19 00:00:00 +0900

Next, let's check how long a month is.

irb(main):002:0> 1.month
=> 2592000

irb(main):003:0> 1.month == 30*24*3600
=> true

So, it looks like 1.month is 30 days, even in February, right?

Wait, does that mean that if I add 1.month to today (Feb. 19th, remember?), then I won't get March 19th as one would expect?

Let's check:

irb(main):004:0> Date.today + 1.month
=> 2015-03-19

irb(main):005:0> 1.month.since.to_date
=> 2015-03-19

Actually I do...

But you said a month is 30 days, and I'm pretty sure that if I add 30 days to Feb. 19th, I won't get March 19th...

Right:

irb(main):006:0> Date.today + 30.days
=> 2015-03-21

irb(main):007:0> 30.days.since.to_date
=> 2015-03-21

So what's up? Is 1.month equal to 30 days, or to 28?

The answer to that is "it depends", obviously.

Understanding how it's working

As we found out one month ago with Ben, 1.day is not an Object. 1.month follows the same pattern, it is an instance of ActiveSupport::Duration, which allows it to behave interestingly.

As seen in Ben's post, ActiveSupport's Date and Time calculations are defined in a couple of files, and we'll focus here on the Date#+ method:

activesupport/lib/activesupport/coreext/date/calculations.rb#L96

def plus_with_duration(other) #:nodoc:
  if ActiveSupport::Duration === other
    other.since(self)
  else
    plus_without_duration(other)
  end
end
alias_method :plus_without_duration, :+
alias_method :+, :plus_with_duration

When adding something to a Date using the + operator, if the right operand is an instance of ActiveSupport::Duration, then the calculation is delegated to the method ActiveSupport::Duration#since, which itself calls #sum.

activesupport/lib/active_support/duration.rb#L90

def sum(sign, time = ::Time.current) #:nodoc:
  parts.inject(time) do |t,(type,number)|
    if t.acts_like?(:time) || t.acts_like?(:date)
      if type == :seconds
        t.since(sign * number)
      else
        t.advance(type => sign * number)
      end
    else
      raise ::ArgumentError, "expected a time or date, got #{time.inspect}"
    end
  end
end

I don't understand why the :seconds case is treated separately (it looks like it would work as well with the else code), but the important line is t.advance(type => sign * number). Long story short,

time + 1.month

is equivalent to:

time.advance(:months => 1)

Note that, at no point, the value of 1.month was converted in days, or in seconds. It was represented as a "quantity of 1, on the month unit", all along. Now if we jump back to Date#advance definition

activesupport/lib/activesupport/coreext/date/calculations.rb#L110

def advance(options)
  options = options.dup
  d = self
  d = d >> options.delete(:years) * 12 if options[:years]
  d = d >> options.delete(:months)     if options[:months]
  d = d +  options.delete(:weeks) * 7  if options[:weeks]
  d = d +  options.delete(:days)       if options[:days]
  d
end

Advancing a date one month will use Date#>> operator, which, according to the Ruby documentation:

returns a date object pointing n months after self

There you have it! At no point, from beginning to end, was a number of days, or seconds, involved.

Consequences

Well, the consequences of such behavior are multiple, some come very handy, while others can be dangerous.

First of all we can admit that it's pretty cool we don't have to worry about the number of days in a month when adding a number of months to a date.

Problems arise when using the same expression in the same piece of code, but this expression ends up having different logical values. Here's a real-life example from the code I'm working on at the moment.

Let's consider a simple Article model, that has a #valid_until attribute.

class Article < ActiveRecord::Base
  DEFAULT_VALIDITY = 1.month

  after_create :set_valid_until
  def set_valid_until
    self.update(valid_until: Time.now + DEFAULT_VALIDITY)

    # Schedule a Sidekiq worker to unpublish in one month.
    ArticleUnpublishWorker.perform_in(DEFAULT_VALIDITY, self.id)
  end

end

This code is not very pretty, but it'll do. When an article is created:

  • Its valid_until attribute is set to one month in the future.
  • A Sidekiq job is scheduled to unpublish the Article in one month.

(Did you notice how I used the same "one month" expression in the two statements above? That mirrors the code using the same 1.month expression in both places.)

One would expect the job to be triggered around the time the article becomes invalid (ideally when now is article.valid_until or else a few milliseconds to seconds after). That would be right only on 30-day months.

Let's say I create an Article today (Feb. 19th 2015). Its valid_until attribute will be set to Mar. 19th, because as we saw above, adding 1.month to a Date (or a Time) will advance it exactly one month. But the Sidekiq worker is scheduled to be run in 1.month, which, all alone, is always equal to 30 days!

There you have it, you thought you used 1.month consistently and that dates would match, but you're getting a 2-day shift between the time the article becomes invalid, and the time it's actually unpublished.


Update: after checking Sidekiq's code, I believe this behavior is a bug, and filed a pull request trying to solve it: MyWorker.perform_in(1.month) does not always schedule job in one month.


Last one for the fun

Now for the fun, let's consider the two following expressions. ```ruby Time.now + 1.month - Time.now - 30.days

Time.now - Time.now + 1.month - 30.days ```

All I did was reorder the members of a simple arithmetic operation, right? They should have the same results.

Not in Rails!

irb(main):008:0> Time.now + 1.month - Time.now - 30.days
=> -172800.0

irb(main):009:0> Time.now - Time.now + 1.month - 30.days
=> 0.0

(remember, time is still frozen using Timecop)

Considering the explanation above, it will be easy for you to understand why...