Liquid filters: date calculations

Hi,

We are using service requests extensively.  We are using API PUT calls within ticket automators to set the due date on the service request according to a date field selected by the user within the service request.  This is working successfully.

I’d like to be able to set the response due date to one or two days prior to that due date, meaning I need to add -2 days to the due date.  For certain service requests, I’d like to be able to set the due date a week after the creation date, meaning I need to add 7 days to the ticket creation date.  I’ve spent days combing support materials and I can’t get anything to work.  This is my best current guess (I’m adding this as a note to the ticket to test the placeholders/filters):

Ticket created at {{ticket.created_at}}

One day later is {{ ticket.created_at | date_add: 1, "day" }}

It doesn’t work, it just gives me the ticket creation date twice and no errors:

Ticket created at Tue, 2021, Dec 14 at 7:58 AM GMT +1000
One day later is Tue, 2021, Dec 14 at 7:58 AM GMT +1000

I am aware the best way to do this sort of thing is with SLAs, priority matrix etc, but we are building a service desk for a non-tech support desk and their business model is slightly different.  

Anyone have any ideas?