Clause for the completion time in contract?

[Boe Dillard]

I know when I was young, my parents had a house built.   They had a very good contractor but he had some personal issues and ended up taking several months longer than the agreed upon contract price.   As a result he had to pay a penalty/ my parents didn't have to pay as much for the house upon completion.   Is there a legal term for this process?   Is this common?