Sharepoint - add days to date in sharepoint designer

If you are looking for a SharePoint designer workflow solution, then I believe there is a 2 step solution to it.

1. Based on the submitted value of Date of join field calculate resulting induction date. This can be achieved by using Add Time to Date action.

Set the values to match your requirement in action, like add 7 change minutes to Days and lastly to CurrentItem:Date Of Join. Refer below screen shot.

2. Now once you get the result date in a variable from the previous step, We just need to set it to the Date of join field using Set Field in current item action. Refer below screen shot

I hope this is helpful.