Thank you Stefi. That worked. I take it you changed part of the formula from
(H11+10) to (H11,10) to take into account the modifications you made.

Not exactly! I changed
H11+10 to
WORKDAY(H11,10,Z1:Z10)
That's a new function with its own arguments!

I also
presume (tell me if I'm wrong) that when I enter the holildays into Column Z,
I will have to re-do this every year (as the dates for all except NY Day,
Independence Day and Christmas Day change yearly).

Have a look at Chip Pearson's site:

http://www.cpearson.com/excel/holidays.htm

It contains solutions for some floating holidays. If you can find a formula
for determining the date of a specific holiday, enter this formula in the
Holidays range, if not, then yes, you have to update Holidays range each year.

Regards,
Stefi