XSLT Select with select variable?

HI Guys,

Firstly - thank you for taking the time to read my question!!  I have the following lines which I am trying to implement but it is not working.

<xsl:variable name="scp" select="/query/clientdemo/transaction[account = 16 and tr_payment_date &gt; <xsl:value-of select='/query/clients/month/monthago13'/>"/>

<xsl:value-of select="sum($scp/tr_to_agency)*-1 + sum($scp/tr_to_client)*-1"/>

I then tried this way of going about it, but still no go!!  Could someone please help?

<xsl:variable name="monththirteen" select="/query/clients/month/monthago13"/>
   
 <xsl:variable name="scp" select="/query/clientdemo/transaction[account = 16 and tr_payment_date &gt; $monththirteen"/>

Thank you in advance!!!