Add Rails/DateAndTimeCalculation cop

[r7kamura]

ActiveSupport provides a lot of useful but not well known date and time helpers. I think it would be nice if RuboCop could wisely recommend the use of these helpers.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.
• If this is a new cop, consider making a corresponding update to the Rails Style Guide.

[r7kamura]

Slightly off topic, but I think the following two Cops are inconsistent in their names. They both deal with timezone issues for a particular class, yet one refers to the class and the other to the timezone.

• Rails/Date
• Rails/TimeZone

We migjht want to name as the following:

• Rails/DateTimeZone
• Rails/TimeTimeZone

or merge them into a single Cop:

• Rails/TimeZone

I understand that it is a bit too late to talk about this now. I thought of that when I was considering whether to separate the Rails/DateAndTimeCalculation Cop into the following two Cops:

• Rails/DateCalculation
• Rails/TimeCalculation

The current name "DateAndTimeCalculation" is after DateAndTime::Calculations module in Rails.

[koic]

Please move the def_node_matcher DSLs under constants. These are not published APIs and are not actually called externally. Mixing DSL and private methods is hard for maintainer to read.

[r7kamura]

OK, I'll make that change 👌

I think it is a common belief that methods should be kept private unless necessary, and that in cases where callbacks are realized in template method pattern like this, that callbacks should be defined in a more prominent position, but in this case you say we should do so even if it breaks these expectations, correct?

It is a bit strange that this point is being discussed at the time of the addition of this Cop, since some of the cops should have already implemented it this way. If there is such an implicit rule, it would be nice to have an internal affair cop or at least have some documentation for that.

[koic]

Reviewers don't always notice it, and they don't always point it out. Whether or not reviewers will comment will depend on the degree. Also, I don't want to retroactively change existing code because it is already in a sufficiently maintainable state.

[koic]

Sure, the cop name and role will need time to be considered. Let's think without rushing because there is triage.