How to validation in datetime

[#1]

Thanks for reponship

[fhunth]

You should write some code to calculate it.

Here you have a reference so as to calculate working days. >>> javascript - Find day difference between two dates (excluding weekend days) - Stack Overflow

Then you should set you field:

function setWorkingDays(value){

  NWF$('#'+yourControl).val(value);

}

Of course it is not considering Nintex Holidays

[#3]

how using function to in Form?

Thanks