Ensuring two digits in EXSLT produced time fields

To automatically produce pubdates, I've followed the advice on this page http://www.dpawson.co.uk/docbook/styling/custom.html#d2212e1704.

In the comments, it mentions "Add number formatting to ensure two digits in time fields." - does anyone know how to do that? At the moment we do indeed get a problem that if a time is, say, 14:07, it comes out as "14:7".

Thanks,

P.