Substantial completion is a date, and a lot hangs on it. On most contracts it's when the final draw becomes due less retainage, when the retainage release period starts, when warranties begin, and in many states when the deadline to record a mechanic's lien starts counting. On permitted work it often coincides with the certificate of occupancy or the final inspection. The client can move in; the contractor can bill; the punch list is what remains.
A worked number
A basement finish is substantially complete on March 3; the punch list closes March 17. A contract that releases retainage 30 days after substantial completion pays the $9,400 on April 2. One that releases it 30 days after final completion pays April 16. The difference is two weeks of a contractor's cash, decided by one line in the contract.
The mistake it hides
Never declaring it. Small residential contracts often skip the term, so "done" becomes whatever the client says it is, and the final draw waits on a paint touch-up. Put the definition in the contract, name the date in writing when it arrives, and bill that day.
In Obra
Mark the date on the project and issue the final charge; what's left to collect then shows the retainage and any unbilled change orders as the only open money on the job.