testing dates

If $isodate contains "2004-06-27" and a CDATA attribute
@start-date contains "2004-09-05", what should be the result of:

<xsl:if test="$isodate &lt; foo/@start-date">

It always evaluates to False under Saxon 6.5.2 (ie if I
use &gt; instead of &lt; I get the same result: False). 

Is there something about comparing strings containing digits that I have
missed? Are they being evaluated as expressions 
(eg 2004 minus 6 minus 27)?

///Peter