But I don't see why anyone would spend money for it. Maybe it's just me.
You must have never worked with any non-trivial date calculations
Try writing a function that would calculate how many business days there are between any two arbitrary dates where 'business day' is defined as 'any day that is not Saturday nor Sunday'. No day-by-day iteration allowed. Pure math please.
I'm failing to see why anyone would use this function over PHP's Calendar functions GregorianToJD() and JDToGregorian(). They encode a date into a 'datestamp'. Better than that, Julian Day format can handle fractional days, so you can have 2454145.25 .. and 6 hours later it's 2454145.5. It's a little confusing because .0 is midday, so .25 is 6pm, and .5 is midnight, and .75 is 6am the next day. Easily 'fixable' by simply subtracting .5 from your Julian Day number though.