I can use averages and multiply the difference in days by 5/7 (weekdays/total days in the week). But that won't be 100% correct. Hopefully it isn't too complex to get the correct answer.

If you aren't concerned with country-specific holidays, and simply need the number of Mon-Fri days between two dates, this can currently be achieved using Javascript and the SCRIPT formula function (2.7+).

See attached IOK file which includes the formula and provides a series of test cases.

(If, on the other hand, you needed to factor in public holidays, you would need a much more sophisticated solution, involving lookup files by country.)

I tried this a while back unsuccessfully, copying the script here:

http://partialclass.blogspot.co.uk/2011/07/calculating-working-days-between-two.html

SCRIPT(
`
function workingDaysBetweenDates(startDate, endDate) {

// Validate input
if (endDate < startDate)
return 0;

// Calculate days between dates
var millisecondsPerDay = 86400 * 1000; // Day in milliseconds
startDate.setHours(0,0,0,1); // Start just after midnight
endDate.setHours(23,59,59,999); // End just before midnight
var diff = endDate - startDate; // Milliseconds between datetime objects
var days = Math.ceil(diff / millisecondsPerDay);

// Subtract two weekend days for every week in between
var weeks = Math.floor(days / 7);
var days = days - (weeks * 2);

// Handle special cases
var startDay = startDate.getDay();
var endDay = endDate.getDay();

// Remove weekend not previously removed.
if (startDay - endDay > 1)
days = days - 2;

// Remove start day if span starts on Sunday but ends before Saturday
if (startDay == 0 && endDay != 6)
days = days - 1

// Remove end day if span ends on Saturday but starts after Sunday
if (endDay == 6 && startDay != 0)
days = days - 1

return days;
}

workingDaysBetweenDates(start, end) ;

`,
'start',[Start Date],
'end',[End Date])


I get problems with the javascript results for some reason. To demonstrate this, running the script below returns false!

SCRIPT(
`
start < end
`,
'start',TEXTTODATE('2012-08-01','yyyy-MM-dd'),
'end',TEXTTODATE('2012-08-23','yyyy-MM-dd'))

whereas the code below returns true:

SCRIPT(
`
var start = new Date("August 01, 2012 00:00:01")
var end = new Date("August 23, 2012 00:00:01")

start < end

`)

To get around this, pass the dates as text using date, and convert them in JS to dates using the code above:

SCRIPT(
`
function workingDaysBetweenDates(startDate, endDate) {

// Validate input
if (endDate < startDate)
return 0;

// Calculate days between dates
var millisecondsPerDay = 86400 * 1000; // Day in milliseconds
startDate.setHours(0,0,0,1); // Start just after midnight
endDate.setHours(23,59,59,999); // End just before midnight
var diff = endDate - startDate; // Milliseconds between datetime objects
var days = Math.ceil(diff / millisecondsPerDay);

// Subtract two weekend days for every week in between
var weeks = Math.floor(days / 7);
var days = days - (weeks * 2);

// Handle special cases
var startDay = startDate.getDay();
var endDay = endDate.getDay();

// Remove weekend not previously removed.
if (startDay - endDay > 1)
days = days - 2;

// Remove start day if span starts on Sunday but ends before Saturday
if (startDay == 0 && endDay != 6)
days = days - 1

// Remove end day if span ends on Saturday but starts after Sunday
if (endDay == 6 && startDay != 0)
days = days - 1

return days;
}
var startd = new Date(start)
var endd = new Date(end)

workingDaysBetweenDates(startd, endd) ;

`,
'start',DATETOTEXT([Start Date],'MMMM dd, yyyy hh:mm:ss'),
'end',DATETOTEXT([End Date],'MMMM dd, yyyy hh:mm:ss'))

I'll post the file in the demos section as well.

I wasn't concerned about public holidays, so what you provided Steve was sufficient. Thanks to you both