Features used to implement the example

• Set a field as a function of other fields

Example: Rise priority if due date is less than 3 weeks away

So, I'm trying to set the Priority field to "Critical" if the currently entered Due Date (if the field has a value) is equal to or less than 3 weeks away. I have the second part of this working, using the Ephemeral number 1 field in a post function: The field Priority will be set according to the evaluation of Ephemeral number 1 against the following set of rules: 

[{Due date}!=null AND {Ephemeral number 1}<=21]Critical

I tested this by just using a temporary additional post function which puts a number value I enter into Ephemeral number 1 field, and it works great. So now I just need the fist part, where the current day is subtracted from the Due Date and returns a whole numerical value (# of days), which is then placed into Ephemeral number 1 field. 

You can do it using only post-function Set a field as a function of other fields. You should use the following configuration:

Setting rule used is: [{00012} != null AND {00012} - {00057} <= 21 * {DAY}]Critical

Note that:

• {00012} is code for numeric value of "Due Date"