The number & date field allows a user to store a number value alongside a date.
This field gets saved as a semicolon-separated string, e.g., "4;14-09-2020". To be able to perform calculations with these values, you have to split the string into 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