Using a parameter in Interval

<p align="LEFT"><span lang="en-us"><font face="Arial" size="2">No matter how I try to concatenate, I
can</font></span><spanlang="en-us"></span><span lang="en-us"><font face="Arial" size="2">’</font></span><span
lang="en-us"></span><spanlang="en-us"><font face="Arial" size="2">t seem to get a parameter to be used by INTERVAL in a
function:</font></span><palign="LEFT"><span lang="en-us"><font face="Arial" size="2">CREATE OR REPLACE FUNCTION
Testing(TrailingWeeksint) RETURNS date AS $$</font></span><p align="LEFT"><span lang="en-us"><font face="Arial"
size="2">BEGIN</font></span><palign="LEFT"><span lang="en-us">        <font face="Arial" size="2">RETURN current_date -
INTERVAL(CAST(TrailingWeeks AS varchar) || ' weeks');</font></span><p align="LEFT"><span lang="en-us"><font
face="Arial"size="2">END;</font></span><p align="LEFT"><span lang="en-us"><font face="Arial" size="2">$$ LANGUAGE
plpgsql;</font></span><palign="LEFT"><span lang="en-us"><font face="Arial" size="2">--</font></span><span
lang="en-us"></span><spanlang="en-us"><font face="Arial" size="2">select * from testing(1);</font></span><span
lang="en-us"></span><spanlang="en-us"></span><p align="LEFT"><span lang="en-us"><font face="Arial" size="2">ERROR: 
syntaxerror at or near "CAST" at character 34</font></span><p align="LEFT"><span lang="en-us"><font face="Arial"
size="2">QUERY: SELECT  current_date - INTERVAL (CAST( $1  AS varchar) || ' weeks')</font></span><p align="LEFT"><span
lang="en-us"><fontface="Arial" size="2">CONTEXT:  SQL statement in PL/PgSQL function "testing" near line
2</font></span><spanlang="en-us"></span><span lang="en-us"></span><p align="LEFT"><span lang="en-us"><font face="Arial"
size="2">Ihave tried concatenating it as a declared variable (with and without apostrophes)</font></span><p
align="LEFT"><spanlang="en-us"><font face="Arial" size="2">1 weeks</font></span><p align="LEFT"><span
lang="en-us"><fontface="Arial" size="2">And </font></span><p align="LEFT"><span lang="en-us"><font face="Arial"
size="2">‘</font></span><spanlang="en-us"></span><span lang="en-us"><font face="Arial" size="2">1
weeks</font></span><spanlang="en-us"></span><span lang="en-us"><font face="Arial" size="2">’</font></span><span
lang="en-us"></span><spanlang="en-us"></span><p align="LEFT"><span lang="en-us"><font face="Arial" size="2">With no
success.Any tips?</font></span><p align="LEFT"><span lang="en-us"><font face="Arial" size="2">Many
thanks,</font></span><palign="LEFT"><span lang="en-us"><font face="Arial" size="2">Robert</font></span><span
lang="en-us"></span><spanlang="en-us"></span>