How do I set a due date x number of days after another task has been completed?

[Domi]

Task A is completed on September 15th. Which formula do I need to use to set the due date of Task B 5 business days after?

Thank you,

Dominique

[John Funk]

Hi Dominique - Welcome to the Atlassian Community!

You would start with an automation rule with a trigger of Issue Transitioned to Done. 

There needs to be some type of linkage between the issues. Once you identify that with a Branch,  you would do a new action of Edit Issue and select the Due Date. 

Then in that field you could use the formulate of: 

{{now.plusBusinessDays(5)}}

[Iago Docando]

would task B be something that you can define with rules (only child of A, linked to A throug  a specific linktype, some specific value in a custom field...) or just a hand picked issue?