http://www.w3.org/TR/xslt20/#format-date defines
   format-dateTime( $value    as xs:dateTime?,
                    $picture  as xs:string,
		     $language as xs:string?,
		     $calendar as xs:string?,
		     $country  as xs:string?) as xs:string?
The picture string is fairly regular.
Has anyone produced a relaxng expression which captures it please?

I hadn't, but just took a crack at it. I think

  xsd:token { pattern =
  "(([^\[\]]|\[\[|\]\])+|\[[YMDdFWwHhPmsfZzCE](((\p{Nd}|\p{Nl}|\p{No}|\p{Lu}|\p{Ll}|\p{Lt}|\p{Lm}|\p{Lo})+|N|n|Nn)[to]?)?(,([0-9]+|\*)(-[0-9]+|\*)?)?\])+"
  }

does the job, (Note xsd:token, not xsd:string, as I wrote this
presuming the string to be matched has already been run through
normalize-space().) Not very well tested. See
http://dev.stg.brown.edu/staff/Syd_Bauman/temp/format-dateTime-picture-validate.tgz.

I took note from John Cowan that comments can be used Syd!
I'm ignoring certain values (simplicity).

<element name='doc'>
<oneOrMore>
<element name='dateTime'>
  <data type='string'>
    <param
name='pattern'><!--
Year -->(\[Y(01|0001)?\][^\[]?)<!--
Month -->(\[M(1|01)?\]?[^\[]?)<!--
Day -->(\[D(1|01)?\])?<!--
Hour -->(T\[H[01]{0,2}\]:<!--
Minutes -->\[m(1|01)?\]:<!--
Seconds         -->\[s(1|01)?\]\.<!--
decimal seconds -->\[f(1|01|001)?\])?</param>
  </data>
</element>

<doc>
<dateTime>[Y0001]-[M01]-[D]</dateTime>
<dateTime>[Y]@[M]@[D]</dateTime>
<dateTime>[Y0001]-[M01]-[D1]</dateTime>
<dateTime>[Y]-[M1]-[D1]</dateTime>
<dateTime>[Y]-[M]-[D]T[H]:[m1]:[s].[f]</dateTime>
<dateTime>[Y][M]-[D01]T[H01]:[m01]:[s01].[f001]</dateTime>
<dateTime>[Y][M]-[D01]T[H01]:[m01]:[s01].[f001]</dateTime>

<!-- Errors -->
<dateTime>[Y100][M][D]T[H]:[m]:[s].[f]</dateTime>
<dateTime>[Y][M100][D]T[H]:[m]:[s].[f]</dateTime>
<dateTime>[Y][M][D10]T[H]:[m]:[s].[f]</dateTime>
<dateTime>[Y][M][D10]T[H]:[m10]:[s].[f]</dateTime>
<dateTime>[Y]-[M]-[D01]T[H01]:[m01]:[s0001].[f001]</dateTime>

   YWw for 'Two thousand and ten'
    MNn   for December
    Dlo   for 31st
    DWo   for twentieth

Its for a graph X axis, using dates.
As you see, we only need the Y M D H m s parts
which makes it a little easier.

--
Dave Pawson
XSLT XSL-FO FAQ.
http://www.dpawson.co.uk