The number & date field allows a user to store a number value alongside a date. This field gets saved as a semi-colon separated string, e.g. "4;14-09-2020". To be able to perform calculations with these values, you have to split the string in separate values. For example, to access the number value in the number & date field, you can use:

var input = '{variable}';
var split = input.split(';');
var number = split[0];
number

To access the date in this field, use "split[1]":

var input = '{variable}';
var split = input.split(';');
var date = split[1];
date