hm, it's described in chapter 'Advanced Steps - Attaching Custom Events To Nintex Date Pickers'.

so what's exactly not clear on that?

This is the stage that I think is vague - I'm not sure how or where to apply the "asynchronous event" or what/how to "hijack" the stack...:

The next step is to attach the functions in the scripts to our date picker controls. This is so that when a user selects dates from the popup calendar, an asynchronous event fires and attempts to retrieve the number of weekends and holidays and update the days column appropriately.

To do this, we have to modify the default date picker controls and append our function calls to it's onSelect attribute. After some digging around, I found that the date picker control is initialized by Nintex by taking the nfFillerLoadFunctions stack and popping the first item out, which contains the function to create date picker objects.

Therefore, we can 'hijack' this stack by popping out and discarding the default date picker control, and then inserting our own:

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you can take whole block of code below the text and copy&paste it to the same script file DateFunctions.js

Thanks - I've done that but am unsure how to modify the default date picker to reference the js file?

you need not to do anything, it's modified by the code