XML : XSL: Apply a template after a math operation on element

I must admit, I am a total noob. So, I apologize ahead of time, as I assume what I'm asking is fairly basic. I'm in the early learning phase right now so please bear with me.

Before displaying LocalTimestamp from my XML file, I need to apply the expression LocalTimestamp div (10 * 1000 * 1000) - 11644473600. The problem is trying to apply the following template at the same time.

  <xsl:template match="LocalTimestamp">     <xsl:value-of   select='xs:dateTime("1970-01-01T00:00:00") + @stamp * xs:dayTimeDuration("PT0.001S")'/>  </xsl:template>    

Any advice or suggestions would be greatly appreciated. Thanks!