Calling function for filtering - xslt

Question

This function "karusell:isCurrentDateTimeBetweenDates" never gets called. Why cant i use it for filtering? If a call i alone and store it in a variable it work. The value of the variable is false.
<xsl:variable name="bannerList" select="verticaldata/contents/content[ karusell:isCurrentDateTimeBetweenDates( 'Thursday', '01' , 'Friday', '03') ][position() <= 5]" />
Edit:
How can the string be returned?
The filtering
The function
<xsl:function name="karusell:isCurrentDateTimeBetweenDates">
<xsl:param name="startDay"/>
<xsl:param name="startHour" />
<xsl:param name="endDay"/>
<xsl:param name="endHour" />
<xsl:variable name="currentDateTime" select="current-dateTime() " />
<xsl:variable name="todayIndex" select="karusell:getDayIndex(format-dateTime($currentDateTime , '[F]'))" />
<xsl:variable name="startDayIndex" select="karusell:getDayIndex($startDay)" />
<xsl:variable name="endDayIndex" select="karusell:getDayIndex($endDay)" />
<xsl:variable name="startDate" select= "$currentDateTime - ( $todayIndex - $startDayIndex )*xs:dayTimeDuration('P1D')"/>
<xsl:variable name="endDate" select= "$currentDateTime + ( $endDayIndex - $todayIndex )*xs:dayTimeDuration('P1D')"/>
<xsl:variable name="startDateTime" select="format-dateTime($startDate, concat('[Y0001]-[M01]-[D01]T', $startHour ,':00:00')) cast as xs:dateTime"/>
<xsl:variable name="endDateTime" select="format-dateTime($endDate, concat('[Y0001]-[M01]-[D01]T', $endHour ,':00:00')) cast as xs:dateTime"/>
<xsl:value-of select="$currentDateTime >= $startDateTime and $currentDateTime < $endDateTime" />
</xsl:function>

Answer 1

Use xsl:sequence, not xsl:value-of to return values of the data type your expression has. So replace
<xsl:value-of select="$currentDateTime >= $startDateTime and $currentDateTime < $endDateTime" />
with
<xsl:sequence select="$currentDateTime >= $startDateTime and $currentDateTime < $endDateTime" />
Additionally you can get better error diagnosis if you define the return type of your function with <xsl:function name="pf:foo" as="xs:boolean">..</xsl:function>.