All solutions posted on the website are examples on how you could get
something done in InfoPath. Needless to say, you need to modify them to suit
your own needs. I can impossibly cater for every InfoPath scenario that there
might be out there. :)

In "Calculating date and time differences with custom VBScript code" you
need to modify the calcTimeInMinutes function to use in your own situation.
And since you  are only dealing with dates, you need to strip out the times.
Change the last line of the function to

calcTimeInMinutes = intDateDiffMin

and remove everything between and including

If intDateDiffMin >= 0 Then
...
End If

and you should be good to go.

The second example "Calculating date differences in InfoPath using VBScript
code" states somewhere in the text just after the ISODateToVBDate function to
"Add the following code to the event handler of a field or button". So you
either need to add a button and put the code in its event handler or add an
OnAfterChange event handler for one of the fields on your form. The
ISODateToVBDate function can go anywhere in the form's code as long as it's
not placed inside another function or event handler.
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S.Y.M. Wong-A-Ton