using durations in date.js

My desired end result is to create a duration from a number of seconds and
then pass this duration to seconds to get a nice HH:MM:SS print out.

Is there a better way to do this than follows?

If I try:
    <xsl:variable name="str_seconds" select="date:seconds('P120D')"/>
I get:
  error evaluating JavaScript (line: 1211; msg: 'SyntaxError: missing ;
  before statement' token: 'undefined')

BUT if I try
    <xsl:variable name="str_seconds" select="date:seconds('2003')"/>
I don't get an error.

Am I misreading the documentation?

Thank you for any help you can give me,
Elisabeth Baute

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