Time::DATE_FORMATS

2011-11-13 23:58 #ruby

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"