The new calendar app in this build has an updated interface that matches that of the mail app. With all of the same basic features from the Windows 8 calendar app, the Windows 10 version simply has a refined interface.
If you are not planning on installing the leaked build, and we don't blame you as this release has been less stable than 10049, you can check out the gallery below.
If you have not noticed, Microsoft is in the process of updating every single component of Windows 10 with a new interface. When the OS reaches RTM sometime this summer, it will likely have very few elements, from a visual perspective, carried over from Windows 8.