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Prettier.

In concert with [bug 5089], the template {era} would be 
reserved. It would be used exclusively for a change in era 
(that is, from BC(E) to AD/CE), usually in a date range or near 
the beginning of a new paragraph.

The template should have 3 optional parameters.

{era} will display the preference as "default", "AD", or "CE".

{era|year} will expand to:
+ positive numeric year, 
  "AD [[year]]" or 
  "[[year]] CE"
- negative numeric year, 
  "[[year BC]]" or 
  "[[year BC|year BCE]]"
= text string, 
  "AD [[1st century]]" or 
  "[[1st century]] CE"

Note that {era} is required for AD/CE text, as such an era change 
is expected to be notable.

{era|year|month} will expand to:
+ positive numeric year, 
  "[[month]], AD [[year]]" or 
  "[[month]], [[year]] CE"
- negative numeric year, 
  "[[month]], [[year BC]]" or 
  "[[month]], [[year BC|year BCE]]" 

The "month, year" display will follow the date preferences. 
All parameters will be numeric.

{era|year|month|day} will expand to: 
+ positive numeric year,
  "[[day month]], AD [[year]]" or 
  "[[day month]], [[year]] CE"
- negative numeric year, 
  "[[day month]], [[year BC]]" or 
  "[[day month]], [[year BC|year BCE]]"

The "day month, year" display will follow the date preferences. 
All parameters will be numeric.

Indeed, it might be particularly handy to have the date 
preferences handled during expansion to avoid a second pass 
by the parser?

Bug 5089 implies the usage of a magic word to do this; there's no need for a
separate bug for each little bit of the same request. Marking as a dupe.

*** This bug has been marked as a duplicate of 5089 ***