Weekday Countdown Timer/Calculator

Hi there,
I’m looking for a plugin which allows visitors to my site to work out what date falls 20 working days after another set date, and possibly offers a countdown to the date.
Essentially it’s so when people are owed something “within 20 working days” they can work out when they’re owed it by, and how long until the deadline.

I know this is quite specific (and possibly a bit complicated) but I’d appreciate any help!