patterns

I've done quite a bit of XSL now (since my last post to this list) and I
even managed to get my head around the Muenchian grouping technique, but
I still do not understand patterns fundamentally. I've prodded chapter 6
("Patterns") of Michael Kay's reference book a couple of times but
always found it a bit too much to bite off and I can't seem to chew off
bits at a time. Hopefully I will eventually do enough XSL/Xpath to
actually understand it when I come back and read it again some time.

Why doesn't this work?
<xsl:value-of
select="./ceiling((endODBCDate/@unixTS - startODBCDate/@unixTS) div 60)"
/>

Sablotron says
"pattern is empty"