Para cuando trabajamos con fechas:

time = Time.now # => Thu Jan 18 06:10:17 CST 2015

time.to_formatted_s(:time) # => «06:10»
time.to_s(:time) # => «06:10»

time.to_formatted_s(:db) # => «2015-01-18 06:10:17»
time.to_formatted_s(:number) # => «20150118061017»
time.to_formatted_s(:short) # => «18 Jan 06:10»
time.to_formatted_s(:long) # => «January 18,2015 06:10»
time.to_formatted_s(:long_ordinal) # => «January 18th, 2015 06:10»
time.to_formatted_s(:rfc822) # => «Thu, 18 Jan 2015 06:10:17 -0600»

http://ruby-doc.org/core-1.9.3/Time.html#method-i-strftime