**Edited to post an improved method, as I found in some cases the JavaScript gave odd results.**

This formula gives the same result as the NETWORKDAYS() function in Excel for all the dates I tested:

DECLARE(
ve,DECLARE(de,DATETOTEXT([End Date],'E'),IF(de='Mon',0,de='Tue',1,de='Wed',2,de='Thu',3,de='Fri',4,de='Sat',5,6)),
vs,DECLARE(ds,DATETOTEXT([Start Date],'E'),IF(ds='Mon',0,ds='Tue',1,ds='Wed',2,ds='Thu',3,ds='Fri',4,ds='Sat',5,6)),
5*INTFLOOR(DATEDIFF([Start Date],[End Date])/7) + VALUE(MID('1234555512344445123333451222234511112345001234550', 7*vs + ve + 1, 1))
)

Source of inspiration:
http://stackoverflow.com/questions/1828948/mysql-function-to-find-the-number-of-working-days-between-two-dates


====== Old version, recommended against! ========
Passing dates as dates into a script can cause odd behaviour in JavaScript. This demonstrates passing them as text, then converting in JavaScript to circumvent those issues.

The result is a calculation of the number of business days between two dates.
The script itself is a copy of:
http://partialclass.blogspot.co.uk/2011/07/calculating-working-days-between-two.html