Calculating a rolling count of days between two dates if one is empty

The formula takes without error, which is fantastic. I wasn't getting any values in the target field at first, but it was successful after I had adjusted the destination field to Formula-Numeric in order to give me the number of days instead of Formula-Duration.

Thanks again.

Just out of curiosity - is it typical to set variables and use special characters ($EndDate) when creating a formula? How would you recognize when to use these functions?