Why do you think that writing tests after you have working code is a waste of time?
As already mentioned, having tests for the code at leasts helps spot regressions when you have to modify the code later. But, more over, even if you have a completely functional application, writing tests can help you uncover areas where there are possible misconceptions before they are found in the live app. Even if the app is working, that doesn't mean it's working as it is supposed to, but you may not realize that without taking the time to write tests against the application.
My humble opinion,