Logging in Ruby on Rails in Production Mode

The normal log level in production is info, so debug logs are not shown.
Change your logging to

Rails.logger.info "Year: #{Time.now.year}"

to show it in production.log.

Alternatively (but not a good idea) you can raise the logging level in /config/environments/production.rb:

config.log_level = :debug

Update Rails 4.2:

Now the default debug level in all environments is :debug (as @nabilh mentioned).
If you want you production environment less chattery, you can reset your log level in /config/environments/production.rb to the former :info:

config.log_level = :info

While I believe @martin-m is right that you probably don't want to clutter your logs by using config.log_level = :debug in /config/environments/production.rb, I believe the default logging level in all environments is debug as of Rails 4.2. So debug logs (and all levels up) will be shown in production unless you specify otherwise.

So in response to your question, you can now write:

Rails.logger.debug "Year: #{Time.now.year}" and see the results in /log/production.log.

See here for more detailed documentation. Here is the documentation for Rails 4.1 for comparison.