In SAP Business One 9.3, on any document for posting date,due date,document date we can set as using Arithmetic Operator.
(e.g. '+10' , '-10' , '+2m' (for months), '+1y' for year).
e.g.
+10 Means if system date is 10/09/2018 then if we put in posting date then posting date is 20/09/2018.
- SYSTEM Date:

2."+10" in delivery Date:
3) output: Delivery date = 10/09/2018+10=20/09/2018.
4) for Months:
4.1:
4.2:
Regards,
Tejas P.