Please refer my previous article to learn more about SharePoint Designer 2013 Workflows.
In this blog you will see how to use “Find Interval Between Dates “action in SharePoint Designer 2013.
Description:
Calculates the time interval in minutes, hours, or days between two dates and stores the output in a variable.
In this example, I am trying to find the time interval in days between “Date of Joining” field and the current date and storing the output in “time” number variable. The current item “Notes” field is updated with the output value.
Workflow Design:
Result: