Test a C# class on its own

This question is about C# and VS2013.

I have a very large web application and have created a class that calculates dates, yesterday, last week etc...

The problem I have is I have to build and run the web application each time I want to test the class. I want to be able to run the class on its own and test the outputs, rather than building the entire app each time.

I think unit testing is the correct way to do it. Any good tutes or advice is always appreciated.

Thanks in advance...