I tried multiple ways to get what I wanted but always failed in some other way
I want to set up a card with a due date and a custom field set to a number. I then want to either use a card button or a detect a move to a different list
I wanted to try the following :
- On move / button press - set date custom field "Date" to {cardduedate}-{{%"Field"}}
=> I'm only able to select a time frame to move it by 
- After that I tried to replace the custom field with a start date
On move / button press - set start date to {cardduedate}-{{%"Field"}}

=> can't use {cardduedate} as it will only allow custom fields to be used here - the last thing I tried was to remove the need for the extra due date by edit the Calender Commands to include the calculation each time
move each card due in less than {{%"Field"}} days to list
=> I get an error CANT_PARSE_COMMAND

At this point I'm kinda stumped and I would like to get some ideas form you
PS: I'm not using "Field" as my custom field I just needed a dummy name