How can I calculate Text field without Japan's holidays?

[MISUMI servicedesk]

I have a Calculated Text field about below.

<!-- @@Formula:
if (issue.get("resolutiondate") == null)
return ((new Date()).getTime() - issue.get("created").getTime()) / 1000 / 3600 /24 + " days" ;
else
return (issue.get("resolutiondate").getTime() - issue.get("created").getTime()) / 1000 / 3600 /24 + " days" ;
-->

But this field include Japan's Saturdays-and-Sundays public holidays.

How can I calculate Text field without Japan's holidays?