As a developer you're never done.
Even if you cannot add new functionality to your code in the time left you can (and should) refactor it:
- improve names,
- reduce code duplication,
- split long methods/functions/procedures into shorter ones
- move methods/functions/procedures to new files to apply SRP and/or same level of abstraction principle.
and other stuff like that.
Any of this tasks takes a few seconds by using your IDEs automated refactoring capabilities. And your unittest will guarantee that you did not change the applications behavior as it is currently implemented.
And in the unlikely case you broke something: checkout the last working state from your SCM...