I've tried following XSLT stylesheet upto now, based on your suggestions,

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
B B B B B B B B B B B B

 B  B  B  B  B  B  B  B  B  B  B  B  B exclude-result-prefixes="xs"
 B  B  B  B  B  B  B  B  B  B  B  B  B version="3.0">

B B <xsl:output method="text"/>

B B <xsl:param name="secsSinceEpoch" as="xs:integer"/>

 B  B  <xsl:template match="/">
 B  B  B  B <xsl:variable name="epochDateTime"
select="xs:dateTime('1970-01-01T00:00:00Z') +

B B B B B xs:dayTimeDuration('PT' || $secsSinceEpoch || 'S')"

 B  B  B  B  B as="xs:dateTime"/>
 B  B  B  B <xsl:variable name="diffDTduration" select="current-dateTime()

$epochDateTime" as="xs:dayTimeDuration"/>
 B  B  B  B <xs:value-of select="$diffDTduration"/>
 B  B  </xsl:template>

</xsl:stylesheet>

When I invoke above stylesheet, as follows (using Saxon 10),

java -classpath saxon-he-10.0.jar net.sf.saxon.Transform -s:trf1.xsl
-xsl:trf1.xsl secsSinceEpoch=1585039138

I get nothing into the output (i.e, <xs:value-of
select="$diffDTduration"/> doesn't print anything). Any thoughts, how
can I proceed further?

But there looks to be a valid value within the variable,B 'epochDateTime'.

let $epochDateTime := xs:dateTime('1970-01-01T00:00:00Z') +
xs:dayTimeDuration('PT' || 1585039138 || 'S')
return
current-dateTime() - $epochDateTime

I am currently not sure why your code using those expressions in XSLT
doesn't output anything, I will try to investigate and let you know.