Yeah, inventing an API and implementation of a date/time/calendar library from scratch without an existing spec, implementation, tests or experience will surely be totally bug-free.
I'm not saying it'll be bug-free. My point is that it'll have exactly the same bugs on either platform. This may be preferable to having different bugs on different platforms.
Different bugs on different platforms are like an early Christmas present:
You don't even need to write tests which include your "expectations" of the result anymore.
Just call the code with randomized inputs for both platforms, and if there is a difference, you found a bug.