Returns the number of working days between two specified dates.
networkdays( starting date, ending date, [holidays] )
starting_date: (Date) The starting date.
ending_date: (Date) The ending date.
holidays: (Date) A list of holidays not counted as workdays.
Integer
An array of holidays may be given by enclosing them in braces, such as {"12/25/2004","12/31/2004"}.
networkdays(date(2011,12,13),date(2011,12,20)) returns 6
