You don't want to use the same method that does the formatting to verify that the value is being formatted properly because it's not clear what you're testing here. In TDD, you write the test first to describe how you want the app to behave. In this case, given an `updated_at` field with the value `2013-07-10T06:36:42Z`, you want the date to be formatted like this: `Wednesday, 10 July 2013 at 6:36:42 PM`. 

Once you settle on the date format you want to to be displayed in the view, you write the test first that expects the date to be displayed as `Wednesday, 10 July 2013 at 6:36:42 PM`, then you write the code to make the test pass. If you don't have a specific expected value in the test, you'll never know if the app behaves as it should (besides checking the view manually). By formatting the value with the same method in the test, your tests will always pass, but the actual behavior might not be what is expected. 

If later, you want to change the date format, and all you do is change the method, if the test is using the method, the test will automatically pass, but you haven't tested it! Once you decide you want to change the date format, the very first thing you should do is update the test to match the expected outcome. Then, change the method. Makes sense?