Even though the return value is not readable, it can still be evaluated.

Code:

$Date1 = "2006/10/01 20:00:20"
$Date2 = "2006/10/01 20:00:21"

$rc = fnDateDiff($Date1, $Date2, "d")

"9 seconds is the minimum interval that is 'readable' when calculating an" ?
"interval with 'd'." ?
"Less than 9 seconds? " Iif($rc > 0.000104166567325592,"False","True") ?
$rc ? 
@ERROR ' : ' @SERROR ?

? "Press any key to continue." Get $

I'm open to suggestions, but changing the output to return '0' just to make it readable could return undesired results.