anchor.diff:  
   `weekyear` or `strict_weekyear`::
   
  -    A formatter for a four digit weekyear.
  +    A formatter for a four digit weekyear: `xxxx`.
   
   `weekyear_week` or `strict_weekyear_week`::
   
  -    A formatter for a four digit weekyear and two digit week of weekyear.
  +    A formatter for a four digit weekyear and two digit week of weekyear:
  +    `xxxx-'W'ww`.
   
   `weekyear_week_day` or `strict_weekyear_week_day`::
   
       A formatter for a four digit weekyear, two digit week of weekyear, and one
  -    digit day of week.
  +    digit day of week: `xxxx-'W'ww-e`.
   
   `year` or `strict_year`::
   
  -    A formatter for a four digit year.
  +    A formatter for a four digit year: `yyyy`.
   
   `year_month` or `strict_year_month`::
   
  -    A formatter for a four digit year and two digit month of year.
  +    A formatter for a four digit year and two digit month of year: `yyyy-MM`.
   
   `year_month_day` or `strict_year_month_day`::
   
       A formatter for a four digit year, two digit month of year, and two digit
  -    day of month.
  -
  +    day of month: `yyyy-MM-dd`.
  +
  
positive.code: 
  `hour` or `strict_hour`::
  
      A formatter for a two digit hour of day.
  
  `hour_minute` or `strict_hour_minute`::
  
      A formatter for a two digit hour of day and two digit minute of hour.
  
  `hour_minute_second` or `strict_hour_minute_second`::
  
      A formatter for a two digit hour of day, two digit minute of hour, and two
      digit second of minute.
  
  `hour_minute_second_fraction` or `strict_hour_minute_second_fraction`::
  
  
negative.code: 
  `date_hour` or `strict_date_hour`::
  
      A formatter that combines a full date and two digit hour of day:
      `yyyy-MM-dd'T'HH`.
  
  `date_hour_minute` or `strict_date_hour_minute`::
  
      A formatter that combines a full date, two digit hour of day, and two
      digit minute of hour: `yyyy-MM-dd'T'HH:mm`.
  
  `date_hour_minute_second` or `strict_date_hour_minute_second`::
  
      A formatter that combines a full date, two digit hour of day, two digit
      minute of hour, and two digit second of minute: `yyyy-MM-dd'T'HH:mm:ss`.
  
  `date_hour_minute_second_fraction` or `strict_date_hour_minute_second_fraction`::
  
  