[xsl] Coding aroung a "Cannot convert zero-length string to an integer"

<xsl:function name="ck:excel-serial-date" as="xs:string?">
  <xsl:param name="input-date" as="xs:string?"/>
  <xsl:value-of select="if($input-date = '') then '' else 
xs:string(xs:integer(translate(xs:string(xs:date($input-date)-xs:date('1900-01-01')),'PD','')))"/>
</xsl:function>

This is a function I wrote (am writing?) to compute an integer that represents 
the a date to Excel 2000.

There may or may not be a value in the input document that corresponds to a 
cell in the output. In that case, I want the function to return an empty string.

So you can see here that I tell the function to expect a string, and that an 
empty string (zero-length string) is acceptable input.

I also tell it that it should return a string, and that a zero-length string is 
acceptable output.

In the body of the function, I state that if the input is a zero-length string, 
it should return a zero-length string, otherwise it is to compute the number of 
days between January 1, 1900 and the input date, convert the result to a 
string, and return that.

Anyway, that's what I thought I was doing, but when I attempt a transformation, 
I get the following error message:

"Fatal Error! Cannot convert zero-length string to an integer"

I thought that this fragment "if($input-date = '') then '' ..." would 
short-circuit any attempt to convert a zero-length string to an integer, but 
apparently I have misapprehended something key here. Can anyone point out where 
I've gone wrong and suggest a fix?

Thanks.

-- 
Charles Knell
cknell(_at_)onebox(_dot_)com - email

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