My basic XML is this:

<Root>
	<Child1>
		<Dates>
			<StartDate>..here is a date/time</StartDate>
			<EndDate>..here is a date/time</EndDate>

<xsl:call-template name="totalHours">
	<xsl:with-param select="Dates/DateSpan/StartDate"

Your XML doesn't have a DateSpan. If this is an omission from
the mail then...

          <xsl:call-template name="totalHours">
            <xsl:with-param name="start" select="$startDate[position()

                                    ^^^^^
This should probably be "startDate".