subscription application

I have created an application that will calculate a hair color formula. My application is in javascript and html. Now my dilemma, I would like to be able to have either a 30 day trial and/or a subscription based version of this software. When someone downloads the software it loads on their computer they simply enter some information and their "formula" is displayed in a browser window. I have tried numerous ways to write a 30 day trial and I have not been happy with any of them. Since there is nothing like this on the market I have come up against some skepticism and therefore I would like to offer a 30 day trial as well as a subscription based form of this software. If you need to see the software please let me know. Thanx Marc