Another approach is to have a document months.xml

<months>
<month name='January' abbr='jan' num='01'/>
<month name='February' abbr='feb' num='02'/>
  ...
</months>

then

document('months.xml')/months/month[@abbr=lower-case($mon)]/@num

(or instead of a separate document you can put the lookup table in a global variable in the stylesheet; but with XSLT 1.0 this relies on the node-set() function.)

   <hank:months>
     <month name='January' abbr='jan' num='01'/>
     <month name='February' abbr='feb' num='02'/>
     ...
   </hank:months>

--
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