I'm talking about if I assume that I will sell the long call at the same time the short call expires or is exercised or is bought back. I need to run this through amibroker...
As suggested earlier, you should just allocate a VERY small amount of money and actually do some trades. It won't take more than a month or two to realize how quickly and easily calendars can lose money.
We could tell you how and why it's not fool proof, but that's not the best way to learn. The best way to learn option trading is to get in there, again with VERY small allocations and learn from trial and error.
Very often the people who are willing and eager to "teach" you, are the very same people who don't know how to structure profitable trades and manage them.
In your case, error will very likely come quickly, and you will begin to develop an understanding and turn it into personal trading rules and systematic strategies that do work in the future.
Good luck sir !