Time::DATE_FORMATS
Time::DATE_FORMATS[:simple_time] = lambda {|time| time = time.to_datetime; "#{time.hour}:#{sprintf('%02d', time.minute)}" }
Time::DATE_FORMATS[:simple_datetime] = lambda {|time| time = time.to_datetime; "#{time.month}/#{time.day} #{time.hour}:#{sprintf('%02d', time.minute)}" }
user.created_at.to_s(:simple_time) #=> "0:00"
user.created_at.to_s(:simple_datetime) #=> "11/14 0:00"