cheat sheets.

$ command line ruby cheat sheets
Ruby 1.8.7
  %a - The abbreviated weekday name (“Sun”)
  %A - The  full  weekday  name (“Sunday”)
  %b - The abbreviated month name (“Jan”)
  %B - The  full  month  name (“January”)
  %c - The preferred local date and time representation
  %d - Day of the month (01..31)
  %H - Hour of the day, 24-hour clock (00..23)
  %I - Hour of the day, 12-hour clock (01..12)
  %l - Hour of the day, 12-hour clock without leading zero (1..12)
  %j - Day of the year (001..366)
  %m - Month of the year (01..12)
  %M - Minute of the hour (00..59)
  %p - Meridian indicator (“AM”  or  “PM”)
  %S - Second of the minute (00..60)
  %U - Week  number  of the current year,
          starting with the first Sunday as the first
          day of the first week (00..53)
  %W - Week  number  of the current year,
          starting with the first Monday as the first
          day of the first week (00..53)
  %w - Day of the week (Sunday is 0, 0..6)
  %x - Preferred representation for the date alone, no time
  %X - Preferred representation for the time alone, no date
  %y - Year without a century (00..99)
  %Y - Year with century
  %Z - Time zone name
  %% - Literal “%” character

   t = Time.now
   t.strftime("Printed on %m/%d/%Y")   #=> "Printed on 04/09/2003"
   t.strftime("at %I:%M%p")            #=> "at 08:56AM"

Ruby 1.9
  %a - The abbreviated weekday name (“Sun”)
  %A - The  full  weekday  name (“Sunday”)
  %b - The abbreviated month name (“Jan”)
  %B - The  full  month  name (“January”)
  %c - The preferred local date and time representation
  %d - Day of the month (01..31)
  %e - Day of the month,
          a single digit is preceded by a space (range ' 1' to '31')
  %F - Equivalent to %Y-%m-%d (the ISO 8601 date format)
  %H - Hour of the day, 24-hour clock (00..23)
  %I - Hour of the day, 12-hour clock (01..12)
  %j - Day of the year (001..366)
  %L - Millisecond of the second (000..999)
  %m - Month of the year (01..12)
  %M - Minute of the hour (00..59)
  %N - Fractional seconds digits, default is 9 digits (nanosecond)
          %3N  millisecond (3 digits)
          %6N  microsecond (6 digits)
          %9N  nanosecond (9 digits)
          %12N picosecond (12 digits)
  %p - Meridian indicator (“AM”  or  “PM”)
  %P - Meridian indicator (“am”  or  “pm”)
  %s - Number of seconds since 1970-01-01 00:00:00 UTC.
  %S - Second of the minute (00..60)
  %U - Week  number  of the current year,
          starting with the first Sunday as the first
          day of the first week (00..53)
  %W - Week  number  of the current year,
          starting with the first Monday as the first
          day of the first week (00..53)
  %w - Day of the week (Sunday is 0, 0..6)
  %x - Preferred representation for the date alone, no time
  %X - Preferred representation for the time alone, no date
  %y - Year without a century (00..99)
  %Y - Year with century
  %z - Time zone offset from UTC (+/-0000, e.g. EST = -0500)
  %Z - Time zone name
  %% - Literal “%” character

   t = Time.now                        #=> 2007-11-19 08:37:48 -0600
   t.strftime("Printed on %m/%d/%Y")   #=> "Printed on 11/19/2007"
   t.strftime("at %I:%M%p")            #=> "at 08:37AM"

Controlling leading padding in Ruby 1.9
  These codes work with any format code. %I is used as an example.
  %-I - remove any leading zeroes or spaces (“1”)
  %_I - force a leading space (and no leading zeroes) if necessary (“ 1”)
  %0I - force a leading zero (and no leading spaces) if necessary (“01”)