If you are including the wiring of the UI into the application in that time I'm impressed, however you still will need to do another 3 major platforms.

And you'll need to have a designer look them over or design them in the first place for each platform, and write documentation for each platform, and test each platform, etc...

In many cases I won't argue that a native app won't be better for the user in many ways, but you can't argue that it's faster to re-create, document, and test your UI 5 different times.