Add new Rails/RelativeDateGrammar cop

This PR adds new `Rails/RelativeDateGrammar` cop.

It checks whether the word orders of a relative dates
are grammatically easy to understand.

```ruby
# bad
tomorrow = Time.current.since(1.day)

# good
tomorrow = 1.day.since(Time.current)
```

Author: aeroastro

Diff for: changelog/new_add_rails_relative_date_grammar_cop

@@ -0,0 +1 @@
+* [#1106](https://github.com/rubocop/rubocop-rails/pull/1106): Add new `Rails/RelativeDateGrammar` cop. ([@aeroastro][])

Diff for: config/default.yml

@@ -861,6 +861,12 @@ Rails/RelativeDateConstant:
   VersionAdded: '0.48'
   VersionChanged: '2.13'
 
+Rails/RelativeDateGrammar:
+  Description: 'Prefer ActiveSupport::Duration as a receiver for a relative date like `1.day.since(Time.current)`.'
+  Enabled: pending
+  Safe: false
+  VersionAdded: '<<next>>'
+
 Rails/RenderInline:
   Description: 'Prefer using a template over inline rendering.'
   StyleGuide: 'https://rails.rubystyle.guide/#inline-rendering'

Diff for: lib/rubocop/cop/rails/relative_date_grammar.rb

@@ -0,0 +1,59 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    module Rails
+      # Checks whether the word orders of relative dates are grammatically easy to understand.
+      #
+      # @safety
+      #   This cop is unsafe because it avoids strict checking of receivers' types,
+      #   ActiveSupport::Duration and Date(Time) respectively.
+      #
+      # @example
+      #   # bad
+      #   tomorrow = Time.current.since(1.day)
+      #
+      #   # good
+      #   tomorrow = 1.day.since(Time.current)
+      class RelativeDateGrammar < Base
+        extend AutoCorrector
+
+        MSG = 'Prefer ActiveSupport::Duration#%<relation>s as a receiver ' \
+              'for relative date like `%<duration>s.%<relation>s(%<date>s)`.'
+
+        RELATIVE_DATE_METHODS = %i[since from_now after ago until before].to_set.freeze
+        DURATION_METHODS = %i[second seconds minute minutes hour hours
+                              day days week weeks month months year years].to_set.freeze
+
+        RESTRICT_ON_SEND = RELATIVE_DATE_METHODS.to_a.freeze
+
+        def_node_matcher :inverted_relative_date?, <<~PATTERN
+          (send
+            $!nil?
+            $RELATIVE_DATE_METHODS
+            $(send
+              !nil?
+              $DURATION_METHODS
+            )
+          )
+        PATTERN
+
+        def on_send(node)
+          inverted_relative_date?(node) do |date, relation, duration|
+            message = format(MSG, date: date.source, relation: relation.to_s, duration: duration.source)
+            add_offense(node, message: message) do |corrector|
+              autocorrect(corrector, node, date, relation, duration)
+            end
+          end
+        end
+
+        private
+
+        def autocorrect(corrector, node, date, relation, duration)
+          new_code = ["#{duration.source}.#{relation}(#{date.source})"]
+          corrector.replace(node, new_code)
+        end
+      end
+    end
+  end
+end

Diff for: lib/rubocop/cop/rails_cops.rb

@@ -97,6 +97,7 @@
 require_relative 'rails/reflection_class_name'
 require_relative 'rails/refute_methods'
 require_relative 'rails/relative_date_constant'
+require_relative 'rails/relative_date_grammar'
 require_relative 'rails/render_inline'
 require_relative 'rails/render_plain_text'
 require_relative 'rails/request_referer'

Diff for: spec/rubocop/cop/rails/relative_date_grammar_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe RuboCop::Cop::Rails::RelativeDateGrammar, :config do
+  it 'accepts ActiveSupport::Duration as a receiver (ActiveSupport::Duration#since)' do
+    expect_no_offenses(<<~RUBY)
+      yesterday = 1.day.since(Time.current)
+    RUBY
+  end
+
+  it 'registers an offense for Date(Time) as a receiver (ActiveSupport::TimeWithZone#ago)' do
+    expect_offense(<<~RUBY)
+      last_week = Time.current.ago(1.week)
+                  ^^^^^^^^^^^^^^^^^^^^^^^^ Prefer ActiveSupport::Duration#ago as a receiver for relative date like `1.week.ago(Time.current)`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      last_week = 1.week.ago(Time.current)
+    RUBY
+  end
+
+  it 'registers an offense when a receiver is presumably Date(Time)' do
+    expect_offense(<<~RUBY)
+      expiration_time = purchase.created_at.since(ticket.expires_in.seconds)
+                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Prefer ActiveSupport::Duration#since as a receiver for relative date like `ticket.expires_in.seconds.since(purchase.created_at)`.
+    RUBY
+
+    expect_correction(<<~RUBY)
+      expiration_time = ticket.expires_in.seconds.since(purchase.created_at)
+    RUBY
+  end
+end