Counter what's the best expression to use

I really think we may be encountering the X Y Problem; thank you Peter Lewis once again for this insight.  Let's take a step back from your planned implementation and not just solve the implementation, but solve the underlying problem.

What is your business problem?  It sounds like your business problem is that you want to have the task automatically reject if no one picks up the task for 12 days.  There's a very simple way to do that which is built into tasks themselves.  It's called an Exception.  In your case, if you're trying to do what I think you're trying to do, you would simply set up a timer expression, and tell the task to move on after 12 days.

Please see if this is helpful:  https://docs.appian.com/suite/help/22.1/Process_Node_and_Smart_Service_Properties.html#exceptions-tab

If not, you can provide more details of what you're ultimately trying to achieve, and we might have an even better approach to the problem.

I really think we may be encountering the X Y Problem; thank you Peter Lewis once again for this insight.  Let's take a step back from your planned implementation and not just solve the implementation, but solve the underlying problem.

What is your business problem?  It sounds like your business problem is that you want to have the task automatically reject if no one picks up the task for 12 days.  There's a very simple way to do that which is built into tasks themselves.  It's called an Exception.  In your case, if you're trying to do what I think you're trying to do, you would simply set up a timer expression, and tell the task to move on after 12 days.

Please see if this is helpful:  https://docs.appian.com/suite/help/22.1/Process_Node_and_Smart_Service_Properties.html#exceptions-tab

If not, you can provide more details of what you're ultimately trying to achieve, and we might have an even better approach to the problem.

What I need is that fo For both tasks, the 1st and the 2nd task, I need to implement/adjust alerts as follows. Number of days are calculated since the sending of the '1ST TASK' task to the Y (i.e. the deadline for both, the 1 ST and the 2ND tasks together must be 12 days).

You can save the current date (and/or DateTime) at the time of completion of a task into a Process Variable (i.e. "pv!task1CompletionDate"), then do subsequent logic with that in later tasks.  Particularly as it'd be pretty easy to get the number of elapsed days between two dates, and use that to refine your logic in various places as you see fit.