Cassandra,
I second Mark's advice. Here is a picture because the visual always helps me.
You need three tables and Pipeline.
When someone adds a Project Status Change to a Project, the Pipeline will fill in the Stop date and time of the previous Project Status Change field as he described.
Elapsed Time is a Formula Duration field: Stop -Start
A nice benefit to this is you will also see who put in the Project Status Change. I like to make people put a note in there as well that describes why it went from Engineering to Production or what ever your various Project Status values are.
