Hello Pauline,
In order to create a new date column that does a increment/decrement on an existing date column - You could use the DATEADD operation on the compute column function.
SYNTAX: DATEADD( @COLNAME@, INCREMENT/DECREMENT NUMBER, INTERVAL)
The interval can be days, years, months, hours etc.
In your case you would use this formula (Shown in image) to calculate your new date.

I hope this helps!
Thank You,
Akshay