cheat sheets.

(Rails 2.1)

Set available formats in config/environment.rb
  ActiveSupport::CoreExtensions::Time::Conversions::DATE_FORMATS

Date
====
            db:'%Y-%m-%d'   2008-08-20
  long_ordinal:'&proc'      August 20th, 2008
          long:'%B %e, %Y'  August 20, 2008
        rfc822:'%e %b %Y'   20 Aug 2008
        number:'%Y%m%d'     20080820
         short:'%e %b'      20 Aug

DateTime
========
            db:'%Y-%m-%d'   2008-08-20 16:56:21
  long_ordinal:'&proc'      August 20th, 2008 16:56
          long:'%B %e, %Y'  August 20, 2008 16:56
        rfc822:'%e %b %Y'   Wed, 20 Aug 2008 16:56:21 -0600
        number:'%Y%m%d'     20080820165621
         short:'%e %b'      20 Aug 16:56

Time
====
            db:'%Y-%m-%d %H:%M:%S'         2008-08-20 16:56:21
  long_ordinal:'&proc'                     August 20th, 2008 16:56
          long:'%B %d, %Y %H:%M'           August 20, 2008 16:56
        rfc822:'%a, %d %b %Y %H:%M:%S %z'  Wed, 20 Aug 2008 16:56:21 -0600
         short:'%d %b %H:%M'               20 Aug 16:56
        number:'%Y%m%d%H%M%S'              20080820165621
          time:'%H:%M'                     16:56

Here’s the (shortened) table for strftime:
%a  weekday name.
%A  weekday name (full).
%b  month name.
%B  month name (full).
%c  date and time (locale)
%d  day of month [01,31].
%H  hour [00,23].
%I  hour [01,12].
%j  day of year [001,366].
%m  month [01,12].
%M  minute [00,59].
%p  AM or PM
%S  Second [00,61]
%U  week of year (Sunday)[00,53].
w  weekday [0(Sunday),6].
W  week of year (Monday)[00,53].
x  date (locale).
%X  time (locale).
%y  year [00,99].
%Y  year [2000].
%Z  timezone name.